Open a bounded compaction headroom grant

vault-checkpoint-cadence (design D3/D4). A compaction transiently needs roughly 2x a vault's source_bytes — the new checkpoint must land durably before the superseded history is pruned — which the org-pooled quota otherwise refuses for a vault already near its limit. Opens a bounded, TTL'd (~1 hour, hard-coded) grant that raises the org's EFFECTIVE storage limit by exactly granted_bytes while active — never the reported usage. The server measures and caps granted_bytes at the vault's OWN current billed source_bytes; requested_bytes, if given, can only narrow it, never widen it. At most one active grant per project (a partial unique index enforces it); a second open while one is active refuses 409 VAULT_COMPACTION_GRANT_ACTIVE. An expired-but-unswept grant releases its slot on this SAME call rather than waiting on the hourly sweep. Granted bytes are never billed and can never become standing storage.