Delete the empty KNOWN_KA4_VIOLATIONS test scaffold

At reviewed SHA `811d6a267b34f1eba2ed0969dd24ccdda565371b`, `KNOWN_KA4_VIOLATIONS` is an empty placeholder for hypothetical future regressions.

- Its only executable use asserts that its length is zero.
- The ordinary `CASES` table and completeness checks already protect current KA-4 behavior.
- Delete the empty constant, future-facing comment, and tautological assertion; record a real exception only when one exists.

Verification and scope

Exact reviewed locations:

- empty future placeholder and rationale: [`test/ka4-rejection-byte-identity.test.ts:459-468`](https://github.com/Comfy-Org/comfy-multi-player/blob/811d6a267b34f1eba2ed0969dd24ccdda565371b/test/ka4-rejection-byte-identity.test.ts#L459-L468)
- sole executable use: [`test/ka4-rejection-byte-identity.test.ts:493-500`](https://github.com/Comfy-Org/comfy-multi-player/blob/811d6a267b34f1eba2ed0969dd24ccdda565371b/test/ka4-rejection-byte-identity.test.ts#L493-L500)
- current resolved exception history: [`docs/decisions/EXCEPTIONS.md:8`](https://github.com/Comfy-Org/comfy-multi-player/blob/811d6a267b34f1eba2ed0969dd24ccdda565371b/docs/decisions/EXCEPTIONS.md#L8)

Independent verification confirmed the constant has no current behavior and catches no regression unless a developer first edits the placeholder itself. Deleting it does not weaken the table-driven rejection cases or their completeness checks.

Open heads #123, #138, #142, and #143 do not modify the scaffold. Existing #149/#150 and merged #140 are unrelated.

Reviewed SHA: `811d6a267b34f1eba2ed0969dd24ccdda565371b`.

Open test/ka4-rejection-byte-identity.test.ts and inspect the placeholder at lines 459-468 and its assertion at lines 493-500. Remove the empty scaffold and tautological check, then run the KA-4 rejection byte-identity test. Done means the ordinary CASES table and completeness checks still pass without the unused constant.

Written by the indexing model from the issue text.