feat(docs): update README to reflect JSON-LD conversion implementation

- mark task 04 as implemented in the project overview
- add convert_json.py as the entry script for task 04
- clarify output paths for JSON-LD conversion artifacts

common/storage_sizes.py

@@ -0,0 +1,31 @@
+"""Idempotent per-format 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.
+"""
+
+from __future__ import annotations
+
+import logging
+from pathlib import Path
+
+logger = logging.getLogger(__name__)
+
+HEADER = "format,variant,bytes"
+
+
+def update_storage_sizes(bench_dir: Path, fmt: str, variants: list[tuple[str, int]]) -> Path:
+	path = bench_dir / "storage_sizes.csv"
+	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]
+	rows = kept + [f"{fmt},{variant},{nbytes}" for variant, nbytes in variants]
+	bench_dir.mkdir(parents=True, exist_ok=True)
+	with open(path, "w", encoding="ascii", newline="\n") as fh:
+		fh.write(HEADER + "\n" + "\n".join(rows) + "\n")
+	logger.info("storage_sizes.csv: %s -> %s", fmt, ", ".join(f"{v}={b}" for v, b in variants))
+	return path