refactor: extract the compose reader into read.py

[lesnik512]

Extracts the JSON/YAML compose reader out of cli.py into a new compose2pod/read.py deep module with a one-function interface read(text: str, fmt: str) -> Any. The repo's subtlest logic — the YAML-1.2 (Docker) SafeLoader that keeps a bare on a string and reads 1e3 as a float — now has a direct test surface (tests/test_read.py) instead of being reachable only through main(). cli.py returns to argparse glue.

read() raises only UnsupportedComposeError; the JSON path now wraps json.JSONDecodeError as invalid JSON: ... (symmetric with the existing invalid YAML:) — the one intended, observable change. Every parsed document is otherwise identical; the auto fallback (invalid JSON → YAML) is preserved.

The move also retargeted the direct importers a first pass missed — test_cli.py's TestYaml12* classes and tests/conformance/conftest.py called the old cli._load_yaml/_read_compose directly — and promoted the stale _read_compose reference in architecture/supported-subset.md.

Rationale and design: planning/changes/2026-07-15.18-extract-compose-reader.md.

Gates: just test-ci 1273 passed @ 100% coverage (read.py 42/42, cli.py 50/50) · just lint-ci clean · just check-planning OK.

🤖 Generated with Claude Code