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

FABRIC-3.md

@@ -301,6 +301,18 @@ decisions get added here, not to `FABRIC-2.md`. Follow the same discipline `FABR
   kernel code touched this step (host-side test tooling only), so no 3-arch acceptance boot
   needed — single live amd64 QMP-hotplug confirmation is the right verification tier.
 
+  **Correction (Captain Bob, 2026-08-26): drives are not bound to any particular size.**
+  64MB was only ever this fixture's arbitrary test-convenience size, matching
+  `usb-thumbdrive-test.img`'s existing BOT-driver-testing precedent — never a real-world
+  constraint. Audited for anywhere this might have implied otherwise: the actual format
+  (`homeblocks_sig_t`) already carries its own `metadata_devblocks` field and hardcodes no
+  size anywhere, confirmed clean. `scripts/bleach_zuse_img.sh` gained a `--size-mb` override
+  so this was never a hidden assumption baked into the tooling either. The earlier
+  `project_usb_thumbdrive_gpt_layout.md` memory's "16GB reference size" phrasing (already
+  hedged as tentative, but risked reading as a target) corrected to state the point
+  explicitly — the GPT layout's design point is the *proportions* (small metadata partition,
+  everything else block storage), not any absolute size.
+
   **Still open, not attempted:** the mint-then-pin boot-sequence fix itself, the `MINT` word,
   and the Zuse recovery path.