Review follow-ups on the stale-code tick yield gate:
- `_gate_mocks` patches `_current_gateway_code_sha` with `raising=False` so the
base tree (where the symbol does not exist yet) fails only on the two new
invariants instead of erroring the whole module out with AttributeError.
- Note at the pid comparison that `get_running_pid(pid_path=None)` can fall back
to `get_runtime_status_running_pid()`, which derives the pid from the same
record — the equality is tautological in that branch and the real proof there
is `runtime_status_is_stale` + `code_sha`.
- Docstring says what the record actually is: `gateway_state.json` is per-HOME
and last-writer-wins, the pid equality binds it to the lock holder, and the
`--replace` takeover window fails open by design.
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