chore(submodule): convert absolute-zero/ to git submodule pinned to upstream (closes #84) (#89)

## Summary

- Replace the in-tree `absolute-zero/` vendored copy (232 files / ~36k
LOC) with a git submodule pinned at upstream
`hyperpolymath/absolute-zero@7da92b3`.
- Rename proof-debt §(e) **VENDORED** → **SUBMODULE**; refresh wording;
close out the "Open owner question 1: vendoring policy".
- Refresh `.hypatia-ignore` header comment to reflect submodule status
(entries stay as a defensive measure).

## Why

Per the issue body, two siblings of `hyperpolymath/absolute-zero` exist
in this repo's history: one in-tree, one canonical upstream. The issue's
own second status-update comment (2026-05-27 15:01Z) confirmed PR #83's
re-vendor + absolute-zero#65 upstreaming left the trees as 174
byte-identical paths + 0 maa-only files + 7 files lightly
upstream-newer. Disposition (B) submodule conversion is the cleanest
landing.

## Behaviour impact

- **`check-trusted-base.sh`** uses `find .` which walks submodule
contents naturally (verified in
`hyperpolymath/standards/scripts/check-trusted-base.sh:54` —
`PRUNE_PATHS` does not list submodules). The 149/150 marker count is
preserved.
- **No build/CI references** to `absolute-zero/` paths exist outside the
submodule itself (verified via `grep -rln "absolute-zero/"
--include="*.yml" --include="Justfile" .` — only hits are inside the
submodule directory).
- **Existing prose references** to `absolute-zero/src/...`,
`.../tests/...`, etc. (in `EXPLAINME.adoc`, `TEST-NEEDS.md`,
`aletheia/conformance/ECHIDNA-CONFORMANCE.md`) continue to resolve
through the checked-out submodule directory.

## Test plan

- [ ] CI green (submodule fetched on workflow runs)
- [ ] `git submodule update --init absolute-zero` succeeds on a fresh
clone
- [ ] `bash scripts/check-trusted-base.sh .` (from
`hyperpolymath/standards`) still reports the same marker count after
submodule conversion
- [ ] Bump path: `git submodule update --remote absolute-zero` advances
pin cleanly

Closes #84.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "absolute-zero"]
+	path = absolute-zero
+	url = https://github.com/hyperpolymath/absolute-zero.git

.hypatia-ignore

@@ -1,10 +1,12 @@
 # SPDX-License-Identifier: MPL-2.0
 #
-# Estate-policy exemptions for legacy demonstration files vendored under
-# absolute-zero/ and aletheia/ subdirs. These pre-date the estate-wide
-# ReScript→AffineScript migration; they live in this monorepo only (not
-# in the standalone upstream repos) and are demonstration scaffolds, not
-# load-bearing code paths.
+# Estate-policy exemptions for legacy demonstration files under
+# absolute-zero/ (submodule, pinned to hyperpolymath/absolute-zero) and
+# aletheia/ subdirs. These pre-date the estate-wide
+# ReScript→AffineScript migration; the absolute-zero/ entries mirror
+# that repo's own .hypatia-ignore (kept here defensively in case
+# hypatia scans through the submodule boundary). They are
+# demonstration scaffolds, not load-bearing code paths.
 #
 # TODO 2026-Q3: migrate these to AffineScript and drop the exemptions.