fix(proposals): stop payload precheck shadowing dead claim refs

[plind-junior]

What changed

approve() no longer runs the page→claim existence check from
_payload_block_reason; it passes the new skip_dead_claim_refs=True so its
own dead-claim-refs branch is the one that decides. check_approvable keeps
the check unchanged, so the batch vouch approve a b precheck still flags
dangling refs and stays all-or-nothing.

Why

two landed changes collided semantically on test. the propose-time payload
validation added a page→claim existence check to _payload_block_reason, and
approve() runs that precheck before reaching its dead-claim-refs handling.
so a page proposal citing a deleted claim raised a flat ProposalError,
DeadClaimRefsError could never surface through the real entrypoint, the
drop_missing_claims strip path became unreachable, and the jsonl surface
reported invalid_request instead of dead_claim_refs.

three cases in tests/test_dead_claim_refs.py are red on test (and on every
pr based on it, e.g. #599) across py3.11/3.12/3.13.

What might break

nothing on disk. no file moves, no field shape changes. the only behaviour
change is at the approve gate, and it restores the documented contract: a page
citing a missing claim raises DeadClaimRefsError (still a ProposalError, so
existing except ProposalError callers are unaffected) instead of the generic
block, and the dangling ref is still never written.

VEP

not a surface change — restores the already-specified approve behaviour.

Tests

• make check passes locally (lint + mypy + pytest)
• New / changed behaviour has a test — tests/test_dead_claim_refs.py goes
  green again; test_approve_page_update_rejects_stale_claim_ref now pins
  the specific DeadClaimRefsError and asserts the existing page body is
  left untouched
• CHANGELOG.md updated under ## [Unreleased] — fixes unreleased
  breakage introduced on test, nothing user-visible to note

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📥 Commits

Reviewing files that changed from the base of the PR and between 4cea97e and 71b9db0.

📒 Files selected for processing (2)

• src/vouch/proposals.py
• tests/test_storage.py

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