feat(kernel): the fixed screen -- with a real Zuse, the system stick alone can only mint a replacement; the old stick dies (#92, Task 5)

Task 5 of plan 2026-10-29-stick-5, accepted in full, and with it #92:
- amd64, aarch64, riscv64: PARITY:V4_POST tests=538 pass=538 fail=0,
  POST: PASSED; word_count=318 dict_hash=0x8c92508f748f99cf on the three
- the PC stick as built, loaded high: 538/538, the prompt
- make -C v4 test-32: 39 programs, 0 failures

Sessions: fallback, replace, mint_fail, system_pull_real; regressions:
every Task 4 session, genesis, stick_genesis, screen_dark_until_login,
screen_dark_after_logout, kbd_typeahead, emergency. Verdicts in
logs/20261110-075036/verdicts/. The plan's Task 5 ticked; STICK.md 11
increment 5a done; FABRIC-3 XLVIII.17; the SER5 bench steps in
boards/BRINGUP.md 4.2.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GE3SDvQn5EoxdGufk84zz2

docs/v4.0.0/STICK.md

@@ -369,6 +369,7 @@ first and seen to fail:
 5a. **The real Zuse stick (2026-10-29)** (§6a): `MINT-ZUSE` and its confirmation, the GPT and
    identity written to the second stick, the marker's real key; recognition of the real Zuse
    stick; the fixed screen and its R; the old stick refused after a replacement.
+   **Done 2026-11-10** (plan `2026-10-29-stick-5`, Tasks 4-5; `FABRIC-3.md` XLVIII.16-17; #92).
 6. **Pull and replug** (§7), and the flush.
 7. **The splash and progress bar** (§8), with the mkcapsule PNG converter.
 8. **Acceptance moves** to the stick boot for x86_64 (§9).

docs/v4.0.0/plans/2026-10-29-stick-5-proxy-zuse-and-real-zuse.md

@@ -317,7 +317,7 @@ on port 2 and no other drive: a first boot every time, the proxy minted, the pro
 - **After a replacement** the marker's `real` is the new key; the old stick then reads `not this
   system's Zuse stick -- not used`.
 
-- [ ] **Step 1: Failing tests** (amd64; the data stick copy from a `mint_zuse` boot, booted again):
+- [x] **Step 1: Failing tests** (amd64; the data stick copy from a `mint_zuse` boot, booted again):
   - **`fallback`:** no Zuse stick: the four lines; no `ok> ` for 30 s; `3 4 + .` and Enter typed,
     then the Zuse stick added: the prompt, and no `7` in the output before `3 4 + .` is typed
     again after it. Then `R` typed with no stick offered asks for one (the plug-in line); any key
@@ -334,13 +334,13 @@ on port 2 and no other drive: a first boot every time, the proxy minted, the pro
     first (no `detach` refusal in the log), `session ended`, headless; replug the data stick (hers
     still in): she is logged in again, by the remembered-drive check of Task 4.
   - Today: FAIL (the proxy with a real key stays headless; no fixed screen).
-- [ ] **Step 2:** Run each: FAIL as listed.
-- [ ] **Step 3:** Implement.
-- [ ] **Step 4:** They pass; regression: all of Task 4's sessions, `genesis`, `stick_genesis`,
+- [x] **Step 2:** Run each: FAIL as listed.
+- [x] **Step 3:** Implement.
+- [x] **Step 4:** They pass; regression: all of Task 4's sessions, `genesis`, `stick_genesis`,
   `screen_dark_until_login`, `screen_dark_after_logout`, `kbd_typeahead`, `emergency`.
-- [ ] **Step 5:** Suite; full acceptance; commit `feat(kernel): the fixed screen -- with a real Zuse,
+- [x] **Step 5:** Suite; full acceptance; commit `feat(kernel): the fixed screen -- with a real Zuse,
   the system stick alone can only mint a replacement; the old stick dies` with the logs.
-- [ ] **Step 6:** A new PC stick (`make boot_image TARGET=PC`, no `STICK_PAIR`) for the SER5, and
+- [x] **Step 6:** A new PC stick (`make boot_image TARGET=PC`, no `STICK_PAIR`) for the SER5, and
   the bench steps written in `boards/BRINGUP.md`: plug in, boot, the prompt; `MINT-ZUSE` onto the
   second stick; boot with the system stick alone (the fixed screen); with both (the prompt).
 
@@ -368,7 +368,7 @@ Tasks 1-3 accepted and merged (#159, master 594d2f93); #155 merged with them.
 - [x] 5.4 her stick pulled: the fixed screen again (not headless)
 - [x] 5.5 the system stick pulled while hers is in: hers let go first (the chain's tail). 5.3-5.5 written in `sk_v4_identity.c` (`sk_v4_fixed_screen`, `sk_v4_session_end`, `g_real_drive`); build on three ISAs; `fallback` PASSES (logs/20261110-063540, -063758). 5.5 itself is tested by `system_pull_real` in 5.6
 - [x] 5.6 the other sessions: `replace` (R, a new stick, the old one refused), `mint_fail` (blkdebug write errors), `system_pull_real` -- all PASS on amd64 (`real_pull` is `fallback`'s pull); `mint_fail` passed on its rerun after its own check was fixed (it counted the first boot's mint in the second boot's log)
-- [~] 5.7 regressions (running: build/t5/suite57/), full acceptance, FABRIC entry, commit; then the SER5 bench steps in `boards/BRINGUP.md` (Task 5 step 6)
+- [x] 5.7 regressions (all Task 4's sessions, genesis, stick_genesis, the screen and keyboard sessions, emergency: PASS), full acceptance (538/538 on the three and the stick high, word_count=318, dict_hash 0x8c92508f748f99cf, test-32 0 failures); FABRIC-3 XLVIII.17; the SER5 bench steps in `boards/BRINGUP.md` 4.2. **Task 5 accepted -- #92 done.** Original line:, FABRIC entry, commit; then the SER5 bench steps in `boards/BRINGUP.md` (Task 5 step 6)
 
 Working notes: the boot's own words are `sk_v4_words[]` in `kernel/src/v4/sk_v4.c`, served by
 `sk_v4_serve(node, request)` with request = index + 1 (`BYE` is 1). Sticks are added in the harness