Share content-hash across storage backends; drop unused Storage.stat()

[in-jun]

Two related cleanups to the Storage port contract.

• Content hash algorithm is an undocumented contract duplicated across storage backends #19 — The SHA-256 content hash is a cross-backend contract (the engine compares local and remote hashes directly), but each backend hand-rolled its own identical MessageDigest + hex-join. Added a single shared ContentHash.sha256Hex in core-sync (streaming, via okio's HashingSource) and pointed both DirectFileLocalStorage and SmbRemoteStorage at it, so a new backend can't silently diverge on algorithm or encoding. Documented the invariant on Storage.scan and FileMeta.contentHash.
• Storage.move() and Storage.stat() are unused port methods that add implementation burden #21 — Removed Storage.stat(), which had no production caller (only tests, which now use probe()). Kept move(): it is now used in production by ConflictManager for "keep both" resolution, so it is a live method rather than port bloat.

Fixes #19
Fixes #21

[in-jun]

Reviewed the full diff against the surrounding code. Both cleanups land cleanly.

#19 — shared content hash

• ContentHash.sha256Hex centralizes the algorithm correctly: HashingSource.sha256 streamed into blackholeSink(), digest read via ByteString.hex(). The source is consumed and closed inside the helper, so both call sites just hand it a fresh source.
• The output encoding is byte-identical to the previous hand-rolled version — hex() and "%02x".format both emit lowercase hex of the same SHA-256 — so hashes already persisted in the ancestor store stay comparable. No migration hazard, and converged files won't suddenly read as conflicts.
• The SMB handle/stream ordering is right: hashing consumes the whole file, so releasing the InputStream (via the okio Source) before the outer .use closes the handle is safe, and the comment captures why it differs from read(). okio is already a dependency of both core-sync and app.
• hashFile/hashRemote stay live through scan(), so nothing goes dead under allWarningsAsErrors.

#21 — drop stat()

• No production caller remained; the only three call sites were tests, all migrated to probe(). The assertions that moved (mtimeMillis equality, null-on-absent) are semantically preserved, and probe() is the cheaper check since it skips hashing.
• Keeping move() is justified — it is used in production by ConflictManager for keep-both resolution.

Contract docs on Storage.scan and FileMeta.contentHash make the cross-backend invariant explicit, which is the real point of the change. No regressions or scope creep. Looks good to merge.