polish(channels): tighten P4 submit-service contract (405 READ + EDIT/REMOVE target guard)

[jung-thomas]

P4 follow-up — tighten submit-service contract

Optional polish parked from the P4 (#2157) whole-branch final review. Two precision improvements on the ChannelSubmissionService insert-only submit path; no model change, no migration.

Changes

1. Insert-only READ assertion tightened — test/channel-submission-service.test.js previously accepted expect.any(Number) for the READ-blocked case (would silently pass a 500). Now asserts the exact CAP-10 contract: 405 Method Not Allowed. Confirmed empirically, not assumed.
2. EDIT/REMOVE target guard at submit time — before('CREATE') now rejects an EDIT/REMOVE submission with no targetChannel_ID with 400 and target targetChannel_ID, instead of accepting a junk PENDING row that only fails later at approve. Defense-in-depth behind the island's client-side required field; ADD (no target) is unaffected.

Verification

• channel-submission-service.test.js: 5 passed (405 READ + two new 400 guard tests + existing anon-401/stamp tests).
• Related suites unchanged: admin-channel-submissions, channel-submissions-model, admin-channel-submissions-app — 16 passed.
• .cds untouched → no .hdbmigrationtable/csn regen.

Draft — targets DEV per branching policy. Not deployed.