fix(snapshot): refuse markers whose format identity the exporter would refuse

[Sagar-024]

fix(snapshot): refuse markers whose format identity the exporter would refuse

Closes #472.

What changed

verify_dataset_snapshot now pins the marker's format identity at the boundary, immediately after the marker is parsed and before any receipt logic runs. The comparison is deliberately identical to the exporter's replace guard (snapshot.py:618-625) and the writer (:772-773): format must equal hflow-dataset-snapshot and format_version must equal "1", so there is exactly one answer to "is this an HFlow snapshot".

A marker without that identity, or with a future version, raises through the same ValueError the unparsable-marker path uses, so the CLI maps it to exit 2 with no new exit code. A foreign directory that happens to carry an integrity-shaped key can no longer be certified; on this base the repro from the issue returned ok: True for exactly that input.

The maintainer's two notes, applied

• Type strictness: the issue repro wrote "format_version": 1 as a JSON int, and the writer records the string "1". The comparison inherits the export guard's strictness, so the int is refused, and per the request the error message says exactly why: it names the found values, the expected values, and states that the version's type is checked because the writer records it as a string.
• Precedent: version 1 is the only version there has ever been; refusing anything else at the same boundary sets the version-2 precedent (a future verifier decides what a newer marker can partly read, and that choice gets made then, deliberately).

Pre-#401 probe result

The very first exporter (402688e, #132) already wrote both fields unconditionally (writer lines :645-646 of that revision), and every revision since has kept them. So no marker the exporter has ever produced lacks the identity, no valid historical input changes answer, and the pre-#401 no-integrity marker (identity present, integrity absent) still returns the no-receipt UNVERIFIABLE path, not a raise.

Boundary family and sequencing

These three sit on one function's input boundary, and the maintainer asked for one change rather than three where possible:

• fix(snapshot): enforce the receipt inventory content_id at verify time #483 (merged): the inventory content_id gate, built to the maintainer's spec on the issue; this branch is rebased onto it and both gates pass the same snapshots.
• This PR: the format-identity gate. Independent one-gate diff; if the maintainer prefers a single combined boundary PR, this commit rebases cleanly onto fix(snapshot): enforce the receipt inventory content_id at verify time #483's branch since the gates sit at different points of the same function.
• [Bug]: Snapshot verify can read outside the handed directory via receipt paths #469 (VARUN3WARE): receipt-path containment, the last sibling, remains with him.

Gates

15 passed in tests/test_snapshot_verify.py (13 existing + the foreign-marker test + the version test covering both the future-version and int-1 refusals), 31 passed across the verify and dataset-snapshot suites, ruff check / ruff format / ty clean. Mutation run: with the identity gate disabled, exactly the two new raise tests go red while the clean, pre-#401, and moved-root tests stay green; restored, all green. Zero non-ASCII and zero em dashes in changed files.

[kstonekuan]

LGTM, merging.

Pushed two commits. The gate repeated the exporter's replace guard verbatim, so both now share _marker_identifies_dataset_snapshot with their own messages. Then the tests that makes safe: dropping the format-name comparison and disabling the exporter guard both passed before, the second because that guard had no test anywhere.