feat: add postgres instrument to collect the pg_stat_statements dump

[fargito]

Add a postgres instrument that captures each benchmark's SQL, call counts, buffer hits, and query plans from pg_stat_statements, keyed to that benchmark's flamegraph.

Enabled with --instruments postgres --postgres-dsn-env-name <ENV>, where the env var holds a superuser connection string — passed by name, not value, so the DSN never reaches the command line, the config dump, or the uploaded logs (mirroring --mongo-uri-env-name).

The runner observes benchmark boundaries over the instrument-hooks FIFO and, on its own connection, resets pg_stat_statements at each benchmark's start and snapshots + EXPLAIN (GENERIC_PLAN, FORMAT JSON)s at each stop — both in the pre-Ack window, so the SQL runs outside the measured region and never perturbs the walltime/flamegraph measurement. Each snapshot is keyed to the benchmark URI (the same one its flamegraph uses) and written to <profile_folder>/instruments/postgres.json as { benchmarks: [{ uri, queries: [...] }] }, riding along in the uploaded profile archive.

Capture is non-fatal: a connection or query failure is logged and never fails the run. On a URI/snapshot count mismatch the artifact is skipped rather than risk misattributing queries to the wrong benchmark.

Requires Postgres 16+ (for GENERIC_PLAN) and a superuser role; targets walltime runs for now (simulation has no runner-side boundary callback yet).

[codspeed]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 17 untouched benchmarks

---

_{Comparing postgres-instrument (af05a49) with main (37eba9e)}