feat(bench): add SQLite and PostgreSQL wrappers for benchmark queries

- implement SQLite and PostgreSQL wrappers to execute benchmark queries
- update README to reflect task 09 as implemented
- enhance storage size updates to handle (format, variant) replacements
- clarify benchmarking methodology for remote PostgreSQL setup

common/storage_sizes.py

@@ -1,8 +1,10 @@
-"""Idempotent per-format updates to out/bench/storage_sizes.csv.
+"""Idempotent per-(format, variant) updates to out/bench/storage_sizes.csv.
 
 Contract (docs/rules/build-pipeline-tasks.md section 4): columns
-format,variant,bytes; a task re-run REPLACES its own format's rows and never
-touches other formats' rows.
+format,variant,bytes; a task re-run REPLACES its own (format, variant) rows
+and never touches other rows. Replacement is per variant, not per format:
+converters 04-07 own the working-footprint variants while task 09 appends
+archival variants of the same formats.
 """
 
 from __future__ import annotations
@@ -25,12 +27,13 @@ def update_storage_sizes(bench_dir: Path, fmt: str, variants: list[tuple[str, in
 	lock = bench_dir / "storage_sizes.lock"
 	fd = _acquire(lock)
 	try:
+		replaced = {(fmt, variant) for variant, _ in variants}
 		kept: list[str] = []
 		if path.exists():
 			lines = path.read_text(encoding="ascii").splitlines()
 			if lines and lines[0] != HEADER:
 				raise ValueError(f"{path}: unexpected header {lines[0]!r}")
-			kept = [ln for ln in lines[1:] if ln and ln.split(",", 1)[0] != fmt]
+			kept = [ln for ln in lines[1:] if ln and tuple(ln.split(",")[:2]) not in replaced]
 		rows = kept + [f"{fmt},{variant},{nbytes}" for variant, nbytes in variants]
 		with open(path, "w", encoding="ascii", newline="\n") as fh:
 			fh.write(HEADER + "\n" + "\n".join(rows) + "\n")