chore: add vulture dead-code detection and remove findings

[leolara]

What

Adds vulture for dead-code detection, then removes the dead code it found. Split into two commits so the tooling and the removals can be reviewed independently.

Commit 1 — add vulture dead-code detection

• Adds vulture to the lint dependency group and a just deadcode recipe.
• Configures vulture in pyproject.toml to scan src, packages, tests, and the whitelist together (most spec code is exercised only through the test suite, so the tests must be in scope or every test-only-used symbol reads as a false positive).
• Adds a tight vulture_whitelist.py that names each indirectly-used symbol explicitly (framework dispatch, serialization, interpreter protocols) instead of relying on wide glob ignores. Excluded from ruff and ty since it is data, not executable code.
• just deadcode is intentionally not wired into just check: vulture has unavoidable false positives on a Pydantic-heavy codebase, so gating CI on it would be noisy.

Commit 2 — remove dead code flagged by vulture

Removes the six symbols just deadcode reported and that were confirmed unused:

• a write-only last-RPC timestamp on the gossipsub peer state
• a mock response-stream flag that was set but never asserted
• an unused parametrize column in the merkleization tests
• an uncalled mock outbound-stream method
• two uncalled test helpers (a key-file stub writer and an interop finalization waiter)

Three reported symbols were kept as confirmed false positives and recorded in the whitelist instead of removed:

• subscribed — essential event field, compared via dataclass equality in tests
• REAL — constructed via CryptoMode("real") from the --crypto CLI option
• MESSAGE_LENGTH — documented XMSS protocol parameter

Verification

• just deadcode → clean (exit 0)
• just check → passes (lint, format, ty, codespell, mdformat, lock)
• affected unit tests pass

Notes

Draft: the whitelist makes ignores explicit, which means a newly-added Pydantic validator/serializer or fixture will trip vulture until added to vulture_whitelist.py. That is the intended trade-off for tight, greppable ignores.

[tcoratger]

Great!