Clarify: home-blocks thumbdrives are not bound to any particular size

Captain Bob's correction. 64MB (disk/zuse.img, usb-thumbdrive-test.img)
was always just an arbitrary QEMU-test convenience size, never a
real-world constraint -- but the docs and script didn't say so
explicitly, and an earlier memory's "16GB reference size" phrasing
(already hedged as tentative) risked reading as a decided target.

Audited the actual format for hardcoded size assumptions: none found.
homeblocks_sig_t already carries its own metadata_devblocks field,
recording whatever size a real drive's partition actually is.

Changes: scripts/bleach_zuse_img.sh gains a --size-mb override (tested
both the override and the unchanged default); disk/README.md's
zuse.img entry and FABRIC-3.md both now state the point explicitly;
the memory file and its MEMORY.md index line corrected to match (the
GPT layout's design point is the *proportions* -- small metadata
partition, everything else block storage -- not any absolute size).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CXjAPTEKrgY2Mrk25KoLDn

scripts/bleach_zuse_img.sh

@@ -1,5 +1,6 @@
 #!/bin/bash
-# bleach_zuse_img.sh - Reset disk/zuse.img back to pristine/unminted state.
+# bleach_zuse_img.sh - Reset disk/zuse.img (or any home-blocks test image)
+# back to pristine/unminted state.
 #
 # disk/zuse.img simulates the physical Zuse superuser thumbdrive for QEMU
 # testing (FABRIC-3.md, Phase 8 kickoff). "Bleaching" it means restoring the
@@ -8,21 +9,43 @@
 # be exercised repeatedly during development without hand-regenerating the
 # whole image each time.
 #
+# 64MB is only this script's default -- an arbitrary QEMU-test convenience
+# size (matching the existing usb-thumbdrive-test.img precedent for BOT
+# driver testing scale), not a constraint on real home-blocks thumbdrives.
+# The design is not bound to any particular drive size: homeblocks_sig_t's
+# own metadata_devblocks field records whatever size is actually observed,
+# nothing in the format or this repo hardcodes one. Override with --size-mb
+# for a different test size.
+#
 # Deliberately a flat/raw image, not GPT-partitioned, matching the same
 # simplifying assumption homeblocks_sig_check()'s current call site in
 # repl.c uses (sig_start_fblock=0) -- both will need to move to a real
 # GPT-partition-relative offset together, once a GPT parser exists. Not
 # invented ahead of that work here.
 #
-# Usage: scripts/bleach_zuse_img.sh [path]
-#   path  Optional override; defaults to disk/zuse.img relative to repo root.
+# Usage: scripts/bleach_zuse_img.sh [path] [--size-mb N]
+#   path        Optional override; defaults to disk/zuse.img relative to repo root.
+#   --size-mb N Optional override; defaults to 64 (arbitrary test convenience, see above).
 
 set -euo pipefail
 
 REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
-IMG="${1:-$REPO_ROOT/disk/zuse.img}"
+IMG="$REPO_ROOT/disk/zuse.img"
 SIZE_MB=64
 
+while [ $# -gt 0 ]; do
+    case "$1" in
+        --size-mb)
+            SIZE_MB="$2"
+            shift 2
+            ;;
+        *)
+            IMG="$1"
+            shift
+            ;;
+    esac
+done
+
 if [ -e "$IMG" ] && [ ! -f "$IMG" ]; then
     echo "bleach_zuse_img: refusing to overwrite non-regular-file '$IMG'" >&2
     exit 1