docs(cartridges): add Reference implementations subsection (#333)

## Summary

Adds a short `== Reference implementations` subsection at the end of
`cartridges/CARTRIDGE-FORMAT.adoc` pointing implementers at the live
downstream consumer of this format.

The cartridge format here is now consumed by
`hyperpolymath/boj-server-cartridges`, which serves as the canonical
registry (already referenced in `<<Registry and on-demand fetch>>`) and
exercises the schema across every shipped cartridge. New section gives
implementers a worked example to read.

## Rationale

Downstream landed three pieces of machinery worth pointing at from the
spec:

- A SHA-pinned mirror of `cartridge-v1.json` at `schemas/` with
`PINNED-SHA` + `SCHEMA-MIRROR.md` documenting the refresh discipline.
- A zero-dep Deno validator at `tools/validate-cartridges/` (only
`jsr:@std/fs` + `jsr:@std/path` + `jsr:@std/assert`) with `audit` /
`audit-verbose` / `strict` / `test` tasks.
- A blocking CI gate at `.github/workflows/cartridge-schema.yml`; strict
mode went live 2026-06-01 after 139/139 manifests passed.

Subsection is ~28 lines, doesn't restate the spec, and clarifies that
downstreams pin by SHA rather than tracking `main` — bumps follow the
procedure in downstream's `schemas/SCHEMA-MIRROR.md`.

## Test plan

- [ ] AsciiDoc renders cleanly (manual review of the new subsection).
- [ ] No other content modified — diff is a pure append at end-of-file.

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

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

Author: hyperpolymath

cartridges/CARTRIDGE-FORMAT.adoc

@@ -156,3 +156,30 @@ The `tray` UI exposes "Add cartridge source" with optional verification against
 This document is v1. Backwards-incompatible changes will be published as
 `cartridge-v2.json` with a migration ADR. v1 manifests remain consumable
 indefinitely; hosts implementing v2 must accept both.
+
+== Reference implementations
+
+The live downstream consumer of this format is
+`hyperpolymath/boj-server-cartridges`, which serves as the canonical registry
+(see <<Registry and on-demand fetch>>) and exercises the schema across every
+shipped cartridge. Implementers building a new host or validator can read it as
+a worked example.
+
+Key files in that repository:
+
+* `schemas/cartridge-v1.json` — SHA-pinned mirror of `cartridges/cartridge-v1.json`
+  from this repo.
+* `schemas/PINNED-SHA` — exact upstream commit the mirror tracks.
+* `schemas/SCHEMA-MIRROR.md` — in-tree authority documenting the refresh
+  discipline.
+* `tools/validate-cartridges/` — zero-dep Deno validator (only
+  `jsr:@std/fs`, `jsr:@std/path`, `jsr:@std/assert`) with `audit`,
+  `audit-verbose`, `strict`, and `test` tasks.
+* `.github/workflows/cartridge-schema.yml` — CI gate running the validator;
+  strict mode went live 2026-06-01 after 139/139 manifests passed.
+
+Refresh discipline: downstream pins the schema by SHA rather than tracking
+`main`. To consume a new revision of `cartridge-v1.json`, follow the procedure
+in the downstream's `schemas/SCHEMA-MIRROR.md` — bump `PINNED-SHA`, refresh
+the mirrored copy, re-run the validator against every manifest, and only then
+land the bump.