Make the KA-13 statelessness reference resolvable

At reviewed SHA `811d6a267b34f1eba2ed0969dd24ccdda565371b`, multiple compliance surfaces cite `KA-13`, but the authoritative invariant register stops at `KA-12`.

- `test/stateless.test.ts`, the strict ESLint config, the stateless gate, and ADR-021 use `KA-13`.
- `docs/INVARIANTS.md` defines only `KA-1` through `KA-12`, so the linked identifier cannot be resolved.
- Canonicalize one label: register the invariant once, or cite ADR-021 plus the existing portability invariant without inventing another alias.

Verification and scope

Exact reviewed locations:

- unresolved test label: [`test/stateless.test.ts:25`](https://github.com/Comfy-Org/comfy-multi-player/blob/811d6a267b34f1eba2ed0969dd24ccdda565371b/test/stateless.test.ts#L25)
- compliance-rule references: [`.agents/checks/eslint.strict.config.js:32-53`](https://github.com/Comfy-Org/comfy-multi-player/blob/811d6a267b34f1eba2ed0969dd24ccdda565371b/.agents/checks/eslint.strict.config.js#L32-L53)
- gate comment: [`scripts/check-stateless.mjs:3`](https://github.com/Comfy-Org/comfy-multi-player/blob/811d6a267b34f1eba2ed0969dd24ccdda565371b/scripts/check-stateless.mjs#L3)
- ADR definition/use: [`docs/decisions/ADR-021-doc-derived-lamport-clock-store.md:55-90`](https://github.com/Comfy-Org/comfy-multi-player/blob/811d6a267b34f1eba2ed0969dd24ccdda565371b/docs/decisions/ADR-021-doc-derived-lamport-clock-store.md#L55-L90)
- invariant register containing `KA-1` through `KA-12`: [`docs/INVARIANTS.md:13-84`](https://github.com/Comfy-Org/comfy-multi-player/blob/811d6a267b34f1eba2ed0969dd24ccdda565371b/docs/INVARIANTS.md#L13-L84)

Independent verification confirmed this is a broken cross-reference, not an absent concept. The Minimalist concern is duplicated naming without a canonical entry, not the statelessness guard itself.

Open heads #123, #138, #142, and #143 modify an unrelated sentence in `docs/INVARIANTS.md` but do not resolve `KA-13`. Existing #149/#150 and merged #140 are unrelated.

Reviewed SHA: `811d6a267b34f1eba2ed0969dd24ccdda565371b`.

Start by comparing the KA-13 references in test/stateless.test.ts, .agents/checks/eslint.strict.config.js, scripts/check-stateless.mjs, and docs/decisions/ADR-021-doc-derived-lamport-clock-store.md with docs/INVARIANTS.md. Run the stateless test and gate before and after the documentation change. Done means every reference resolves to one canonical invariant or an explicitly cited existing definition, with no duplicated alias.

Written by the indexing model from the issue text.