Milestone 6: hard-refuse flip, scoped to INVALID only -- closes Milestone 6

Before flipping, found CAPSULE_SIG_MISSING (no signature at all) is the
normal state everywhere except this machine -- CI and any other
checkout have no access to the offline key, by design. Refusing on
MISSING the same as INVALID would brick boot everywhere but here.
Decided (on request): enforce ONLY on CAPSULE_SIG_INVALID (a signature
that IS present but doesn't verify -- unambiguous tampering/corruption
evidence). MISSING/NO_ROOT_KEY stay WARN-only permanently.

All three capsule_birth.c call sites now return CAPSULE_RUN_ERR_INVALID
on CAPSULE_SIG_INVALID, after logging the same WARN as before.

Verified on all three architectures, both directions, per the original
rollout commitment: positive case (real signed capsules) reboots clean
with zero warnings on amd64/aarch64/riscv64. Negative case (same
one-byte signature corruption used for the WARN-only proof, on Mama's
own init.4th) now genuinely refuses identically on all three:
"capsule sig: init.4th: INVALID" then "Init: Mama birth FAILED". The
feared "no ok> at all" blast radius didn't materialize -- kernel_main.c
already had graceful error handling for a failed Mama birth (log and
continue, pre-existing code); the kernel reaches a degraded ok> rather
than crashing, on all three architectures. Final acceptance pass (real
signed capsules, tampering reverted) clean on all three.

Milestone 6 is now fully closed except magic-number content-type
detection (shared with Milestone 4, separate scope, not started).
Documented in FABRIC-3.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U14ET9CWAtbQMbYqomKgXd

FABRIC-3.md

@@ -823,6 +823,32 @@ decisions get added here, not to `FABRIC-2.md`. Follow the same discipline `FABR
   magic-number content-type detection (Section U item 14, shared with Milestone 4, not
   started) and the hard-refuse flip (deliberately deferred).**
 
+  **Hard-refuse flip, done 2026-08-26 -- and it landed narrower than the original plan
+  assumed.** Before flipping, found that `CAPSULE_SIG_MISSING` (no signature at all) is the
+  *normal* state on every machine except this one -- CI and any other checkout have no access
+  to the offline key, by design. Refusing on `MISSING` the same as `INVALID` would brick boot
+  everywhere but here, not catch anything real. **Decided (on request): enforce ONLY on
+  `CAPSULE_SIG_INVALID`** (a signature that IS present but doesn't verify -- unambiguous
+  tampering/corruption evidence, safe to refuse on regardless of who's building).
+  `MISSING`/`NO_ROOT_KEY` stay WARN-only permanently, not as a transitional state.
+
+  All three `capsule_birth.c` call sites now `return CAPSULE_RUN_ERR_INVALID` on
+  `CAPSULE_SIG_INVALID`, after logging the same WARN as before. **Verified on all three
+  architectures, both directions, exactly per the original rollout commitment:** the positive
+  case (real signed capsules) reboots clean with zero warnings on amd64/aarch64/riscv64,
+  identical to before this flip. The negative case (the same one-byte signature corruption
+  technique used for the WARN-only proof, on Mama's own `init.4th`) now genuinely refuses --
+  `capsule_birth_mama()` returns the error -- on all three architectures identically:
+  `capsule sig: init.4th: INVALID` followed by `Init: Mama birth FAILED`. **The feared "no
+  ok> at all" blast radius didn't materialize**: `kernel_main.c` already had graceful
+  error handling for a failed Mama birth (log and continue, pre-existing code, not written for
+  this feature) -- the kernel reaches a degraded `ok>` rather than crashing or hanging, on all
+  three architectures. Final acceptance pass (real, correctly-signed capsules, all tampering
+  reverted) clean on all three, zero warnings, conservation intact, real disk untouched.
+
+  **Milestone 6 is now fully closed** except magic-number content-type detection (shared with
+  Milestone 4, genuinely separate scope, not started).
+
 ### From FABRIC-2.md §X, Milestone 7 — Contributor capsules / trust tiers
 
 - [ ] Create the `capsules/contrib/` directory (mechanically trivial, matches existing

capsules/BLOCK_MAP.md

@@ -1,5 +1,5 @@
 # Capsule Block Manifest — Auto-generated
-<!-- Generated by mkcapsule --manifest  2026-08-27T01:40:17Z -->
+<!-- Generated by mkcapsule --manifest  2026-08-27T02:05:00Z -->
 <!-- DO NOT EDIT — re-run mkcapsule --manifest to refresh.  -->
 <!-- Hand-written justifications and immutability notes live  -->
 <!-- in MANIFEST.md alongside this auto-generated index.      -->

include/starkernel/capsule_sig.h

@@ -5,13 +5,17 @@
  * signature/behavior stays untouched; this is a new, additive check
  * called alongside it, not folded into it.
  *
- * Currently WARN-only, not enforced -- see FABRIC-3.md's Milestone 6
- * rollout decision. A bug here has a larger blast radius than most of
- * this project's other checks: a false refusal on Mama's own capsule
- * would mean no `ok>` at all, on any architecture. Land warn-only,
- * prove correct against both a valid and a deliberately-corrupted
- * capsule on all three architectures, then flip to hard-refuse
- * separately.
+ * Enforced ONLY on CAPSULE_SIG_INVALID (2026-08-26, after landing
+ * WARN-only and proving correct on all three architectures against both
+ * a valid and a deliberately-corrupted capsule -- see FABRIC-3.md's
+ * Milestone 6 writeup). CAPSULE_SIG_MISSING and CAPSULE_SIG_NO_ROOT_KEY
+ * stay WARN-only, deliberately: MISSING is the normal state on every
+ * machine without access to the offline signing key (CI, any other
+ * checkout) -- refusing on it would brick boot everywhere but the one
+ * machine that minted the key, not catch anything real. Only INVALID
+ * (a signature that IS present but does not verify) is unambiguous
+ * tampering/corruption evidence, safe to refuse on regardless of who's
+ * building.
  */
 #ifndef STARKERNEL_CAPSULE_SIG_H
 #define STARKERNEL_CAPSULE_SIG_H

src/starkernel/capsule/capsule_birth.c

@@ -416,9 +416,13 @@ CapsuleRunResult capsule_birth_mama(
     CapsuleValidateResult vr = capsule_validate(mama_cap, arena, dir->arena_size, 1);
     if (vr != CAPSULE_VALID) return CAPSULE_RUN_ERR_INVALID;
 
-    /* Milestone 6 (Phase 8): WARN-only signature check, not yet enforced --
-     * see capsule_sig.h's own doc comment for why (blast radius: a false
-     * refusal here means Mama herself never births, no ok> on any arch). */
+    /* Milestone 6 (Phase 8): signature check. Enforced ONLY on INVALID (a
+     * signature that IS present but does not verify -- unambiguous
+     * tampering/corruption evidence). MISSING and NO_ROOT_KEY stay
+     * WARN-only: MISSING is the normal state on every machine without
+     * access to the offline signing key (CI, any other checkout) --
+     * refusing on it would brick boot everywhere but the one machine
+     * that minted this key, not catch anything real. See capsule_sig.h. */
     {
         int idx = (int)(mama_cap - descs);
         CapsuleSigResult sr = capsule_verify_signature(
@@ -426,6 +430,7 @@ CapsuleRunResult capsule_birth_mama(
         if (sr != CAPSULE_SIG_OK) {
             log_message(LOG_WARN, "capsule sig: %s: %s",
                         names[idx].name, capsule_sig_result_str(sr));
+            if (sr == CAPSULE_SIG_INVALID) return CAPSULE_RUN_ERR_INVALID;
         }
     }
 
@@ -488,7 +493,9 @@ CapsuleRunResult capsule_birth_baby(
     CapsuleValidateResult vr = capsule_validate(cap, arena, dir->arena_size, 1);
     if (vr != CAPSULE_VALID) return CAPSULE_RUN_ERR_INVALID;
 
-    /* Milestone 6 (Phase 8): WARN-only, see capsule_sig.h. */
+    /* Milestone 6 (Phase 8): enforced only on INVALID -- see the fuller
+     * comment in capsule_birth_mama() above for why MISSING/NO_ROOT_KEY
+     * stay WARN-only. */
     {
         int idx = (int)(cap - descs);
         CapsuleSigResult sr = capsule_verify_signature(
@@ -496,6 +503,7 @@ CapsuleRunResult capsule_birth_baby(
         if (sr != CAPSULE_SIG_OK) {
             log_message(LOG_WARN, "capsule sig: %s: %s",
                         names[idx].name, capsule_sig_result_str(sr));
+            if (sr == CAPSULE_SIG_INVALID) return CAPSULE_RUN_ERR_INVALID;
         }
     }
 
@@ -634,7 +642,9 @@ CapsuleRunResult capsule_run_experiment(
     CapsuleValidateResult vr = capsule_validate(cap, arena, dir->arena_size, 1);
     if (vr != CAPSULE_VALID) return CAPSULE_RUN_ERR_INVALID;
 
-    /* Milestone 6 (Phase 8): WARN-only, see capsule_sig.h. */
+    /* Milestone 6 (Phase 8): enforced only on INVALID -- see the fuller
+     * comment in capsule_birth_mama() above for why MISSING/NO_ROOT_KEY
+     * stay WARN-only. */
     {
         int idx = (int)(cap - descs);
         CapsuleSigResult sr = capsule_verify_signature(
@@ -642,6 +652,7 @@ CapsuleRunResult capsule_run_experiment(
         if (sr != CAPSULE_SIG_OK) {
             log_message(LOG_WARN, "capsule sig: %s: %s",
                         names[idx].name, capsule_sig_result_str(sr));
+            if (sr == CAPSULE_SIG_INVALID) return CAPSULE_RUN_ERR_INVALID;
         }
     }