chore(schemas): SHA-pin cartridge-v1.json to standards@7c2b815 (#200) (#166)

## Summary

Closes the `SCHEMA-MIRROR.md` pinning ceremony left open after
`hyperpolymath/standards#200` merged 2026-05-26.

Adds `schemas/PINNED-SHA` capturing:
- Canonical commit: `standards@7c2b815`
- File SHA-256:
`033e2fa8cbee5338fbdd73dfd86ac2a3ca76536af31d0ac4a8184f38a6718997`
- Bytes: 5613

Verified 2026-05-30 — local mirror at `schemas/cartridge-v1.json` is
byte-identical to the canonical at the pinned SHA.

## Why

`schemas/SCHEMA-MIRROR.md` documented the pinning ceremony but the
ceremony was never executed. Without a pin file:
- Drift between mirror and canonical is undetectable mechanically.
- Future PRs that bump the mirror could land without an explicit
canonical-SHA reference.

The pin file gives reviewers a single grep target (`PINNED-SHA`) to
verify any future mirror bump is matched by a standards commit.

## Test plan

- [ ] PINNED-SHA SHA-256 verifies against canonical: `git -C
../standards show 7c2b815:cartridges/cartridge-v1.json | sha256sum`
- [ ] Local mirror matches: `sha256sum schemas/cartridge-v1.json`
- [ ] Both print `033e2fa8...`

## Notes

A parallel pin should land in
`hyperpolymath/boj-server-cartridges/schemas/PINNED-SHA` (repo not
cloned locally — separate PR or follow-up).

Refs hyperpolymath/standards#200

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

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

Author: hyperpolymath

schemas/PINNED-SHA

@@ -0,0 +1,23 @@
+# Cartridge schema pin
+
+This file records the canonical-schema pin for `schemas/cartridge-v1.json`.
+
+Pin source:  https://github.com/hyperpolymath/standards (PR #200, merged 2026-05-26)
+Pin commit:  7c2b815
+Pin path:    cartridges/cartridge-v1.json
+Pin SHA-256: 033e2fa8cbee5338fbdd73dfd86ac2a3ca76536af31d0ac4a8184f38a6718997
+Pin bytes:   5613
+
+Local mirror verified byte-identical to standards@7c2b815:cartridges/cartridge-v1.json
+on 2026-05-30. See `SCHEMA-MIRROR.md` for the pinning ceremony rationale.
+
+To re-verify:
+
+    git -C ../standards show 7c2b815:cartridges/cartridge-v1.json | sha256sum
+    sha256sum schemas/cartridge-v1.json
+
+Both must print 033e2fa8cbee5338fbdd73dfd86ac2a3ca76536af31d0ac4a8184f38a6718997.
+
+When `standards` advances `cartridges/cartridge-v1.json` (v1.x minor or v2),
+update both this file and `cartridge-v1.json` (or add `cartridge-v2.json`) in a
+single PR. Do not advance the mirror without advancing this pin.