fix(release): repair the two failures that left v0.4.3 unsigned

[livingstaccato]

v0.4.3 reached PyPI but its GitHub release has 0 assets and no signatures. Two independent failures, one per commit here.

1. SBOM: marker matched as a substring instead of evaluated

scripts/sbom_from_wheel.sh demanded every Requires-Dist name appear in the SBOM, skipping only lines containing the literal "extra ==". That is a hand-rolled evaluation of one marker, blind to every other.

tzdata; sys_platform == "win32" carries no extra, so it was demanded on the ubuntu runner that builds the SBOM — where the marker correctly keeps it uninstalled:

SBOM lists 3 components but is missing 1 of provide-foundation's dependencies (tzdata) -- wrong environment

Two commits in the same release collided: 61f22308 added the closure proof, 74c6ba45 added the first conditional dependency this project has ever had. Neither was wrong alone, and no earlier release could have caught it.

Markers are now evaluated with packaging, against an empty extra (which is what makes extra == "cli" false — no extra was requested when the wheel was installed). The checker runs under the venv's own interpreter, because the only environment whose marker answers mean anything is the one the SBOM describes. packaging is installed after generation; that order is load-bearing, or it would be published as a component of the release.

Verified against a fresh uv build of 0.4.3:

	result
main's script	reproduces the CI failure verbatim, on macOS, same reason
this script	SBOM describes provide-foundation 0.4.3 with 3 dependencies — attrs, structlog, tomli_w, and not packaging
drop structlog from the SBOM, re-check	still fails, exit 1 — the check has not been weakened into always passing

2. Sigstore: signing aborts on a trust root that no longer verifies

Both signing steps pinned the action at v3.0.0 (July 2024), which installs sigstore ~= 3.0. sigstore-python 3.x bundles a TUF root that can no longer verify Sigstore's production root:

tuf.api.exceptions.UnsignedMetadataError: root was signed by 0/3 keys

raised from TrustedRoot.production() before anything is signed. It does not degrade — it aborts, which is why the release ended up with no artifacts attached.

Observed on the v0.4.3 repair dispatch (run 33049593932), which fetched the published artifacts from PyPI successfully and then died on signing.

v3.1.0 moved the action to sigstore-python 4.x for Rekor v2; v3.5.0 is current. rekor-version still defaults to 1 in the action's 3.x series, so bundle format is unchanged and the dist/*.sigstore.json upload glob is unaffected. Still pinned by SHA, with a note at both call sites recording why pinning back below v3.1.0 breaks signing.

After this merges

Re-dispatch release.yml with release_tag: v0.4.3 to attach and sign the bytes PyPI already holds. That path emits no SBOM by design, so v0.4.3 keeps signed artifacts without one; 0.4.4 onward gets both.