Shared state + cache reuse: Postgres backend, adapter-aware caching, state inspection - #10

Summary

Bundle 6 — shared state + cache reuse.

• PostgresStateStore — full StateStore implementation mirroring the SQLite schema (upserts, BIGSERIAL ordering tiebreak, same migrations). The postgres client drives an internal Tokio runtime that cannot block_on inside the outer one, so the client lives on a dedicated worker thread and each trait call ships a closure over a channel — legal from sync and async callers alike.
• --state / PHLO_STATE_URL — postgres:///postgresql:// selects the shared backend; any other value is a SQLite file path (default .phlo/transform/state.db). An explicitly configured location that fails is a hard error — never a silent fallback to local state — and credentials in the URL are stripped before it appears in errors or doctor output.
• Safe cross-environment cache reuse — a same-version record in another environment authorises Cached only when it names the same physical target relation AND was produced by the same adapter; a same-version record failing either check yields build with a new cache_miss reason (detail names the mismatch), and a current-environment record produced by a different adapter rebuilds with adapter_change. model_versions records the producing adapter (best-effort ALTER TABLE migration; NULL rows cannot authorise reuse).
• phlo-transform state — read-only inspection: runs (env filter), show <run-id|prefix> (model/seed/test records), model <name> (version components, target, adapter), promotions. All honour --json. RunSummary gains environment.
• Concurrency — SQLite busy_timeout(5s) so parallel processes sharing a state file wait out lock contention.

Test plan

• cargo fmt --all -- --check clean
• cargo clippy --workspace --all-targets -- -D warnings clean
• cargo test --workspace green — incl. new: cache_reuse_requires_same_physical_target, cache_reuse_requires_same_adapter, cache_reuse_rejects_unrecorded_adapter, materialisation_by_another_adapter_rebuilds, concurrent_sqlite_writers_complete, state_commands_inspect_recorded_runs_on_duckdb, state_flag_selects_sqlite_path, unreachable_postgres_state_url_errors
• Postgres backend covered by postgres_roundtrip_run_and_versions (opt-in via PHLO_TEST_POSTGRES_URL; skips cleanly without a server)