FABRIC-3.md §I.1: (user) console prompt segment, closing the 4.4s->4.3->1.11 chain

Extends the REPL prompt to "[VM name] (user) ok>" (e.g. "[Hera]
(zuse) ok>") per the locked FABRIC.md §4.4s spec, unblocked by this
session's own §I.3/§I.8 identity-tracking work. Adds
capsule_wirebind_attached_username() alongside the existing tracked
VMUuid, and a new sk_print_prompt() helper (repl.c) that all three
prompt call sites now go through -- checks Zuse first, then a WIREBIND
user, prints nothing when neither is attached.

Closes 4.4s, 4.3 (console umbrella), and formally settles 1.11
(dirty-event granularity) as region-based per FABRIC-2.md's own "no
independent path" ruling -- a decision closure only, not an
implementation, so §17.4 (framebuffer heat/decay physics) stays open,
re-scoped precisely: blocked on the dirty-region-tracking mechanism
existing, not on 1.11's decision.

Documents a reported-but-unreproduced terminal defect (§I.9) as a new
punch-list item -- investigated the readline/keyboard-bridge code
paths, found nothing conclusive, needs a live repro with a serial log
before it's actionable.

Verified 3-arch boot to ok> (amd64/aarch64/riscv64, each in the
foreground); logs and DoE CSVs from this session's verification runs
included per this repo's own audit-artifact convention.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019YcT3H2PQeyujrzjqS3Var

include/starkernel/capsule_wirebind.h

@@ -112,6 +112,16 @@ int capsule_wirebind_eject(void);
  */
 void capsule_wirebind_unclean_detach(void);
 
+/**
+ * capsule_wirebind_attached_username - The plain username (no "~user"
+ * registry-name suffix) of whichever identity is currently tracked as
+ * attached, or NULL if none is (FABRIC-3.md §I.1/4.4s -- the `(user)`
+ * console prompt segment reads this). Points into WIREBIND's own
+ * internal storage; valid only until the next attach/eject/detach, same
+ * caveat as console_get_vm_name().
+ */
+const char *capsule_wirebind_attached_username(void);
+
 #endif /* __STARKERNEL__ */
 
 #endif /* STARKERNEL_CAPSULE_WIREBIND_H */