feat: add per-episode LeRobot manifest receipts

[victorwon2001]

Summary

• Upgrade prepared-manifest.json to schema version 3 and record filename, content_id, and size_bytes for every published LeRobot episode.
• Reuse hflow.catalog.content_episode_id() on the staged canonical MCAP instead of introducing another hashing implementation.
• Preserve the existing episodes_converted, dataset, camera, converter-version fields, public list[str] return contract, and manifest-last publication behavior.
• Document the v3 receipt format. The verify command remains deferred as requested and can later consume this schema additively.

Fixes #379

Why

The previous manifest recorded only how many episodes were converted, so a delivered corpus had no per-file identity or size receipt. Recording those values at conversion time provides the required provenance without changing canonical MCAP bytes, CONVERTER_VERSION, transform behavior, or curation identity semantics.

Validation

• uv run pytest -q tests/test_lerobot_converter.py tests/test_transform.py — 57 passed.
• uv run ruff check --fix — passed with no changes.
• uv run ruff format — passed; all 212 files unchanged.
• uv run ty check — passed.
• lychee --no-progress --include-fragments --exclude '^https://github\.com/Hebbian-Robotics/hflow/(issues|security/advisories/new)$' --exclude-path references/mcap-spec.md --exclude-path references/foxglove-CompressedVideo.proto . — 0 link errors.
• uv run pytest -q — 1453 passed, 6 skipped, with one unrelated date-sensitive failure in test_preview_stats_render_timestamps_in_utc_on_a_non_utc_host: on 2026-09-04 the UTC timestamp itself contains the substring -04 that the test forbids.

Checklist

• I added or updated outcome-focused tests for changed business logic.
• I updated documentation for changed behavior, flags, formats, or requirements.
• I ran uv run ruff check --fix, uv run ruff format, and uv run ty check.
• I ran the relevant pytest suite.
• I did not add recordings, generated media, credentials, private URLs, or runtime artifacts.
• I preserved stored-data compatibility or documented an explicit version change.

[github-actions]

👋 Hi @victorwon2001 — thanks for the contribution! To keep starter issues available
for other contributors and give every pull request a real review, we accept
1 open pull request per contributor at a time.

You already have #380 open, so this one is being closed automatically.
Once your open pull request is merged or closed, feel free to reopen this one —
no work is lost.