Reconcile Data 0.3 and Diagnostic 0.2 Python 3.15 scopes - #57

Data 0.3 maintenance candidates fail the exception validator because the previous Polars exception ends before 0.3. Add a separate Data-only Databento exception for >=0.3,<0.4, using current issue 37 evidence. Preserve the old Polars record and Engineer selection unchanged.

Decision 0016 also gives Diagnostic a qualification-pending deferral through its 0.2 line until November 24, 2026. Current core dependencies install and import on Python 3.15, so the obsolete missing-PyArrow-wheel explanation is removed. The existing identifier is retained for caller compatibility. Full native/installed-artifact three-OS qualification, truthful Python bounds and a monthly canary remain required. No numerical, security, artifact or supported Python 3.12-3.14 gate is waived; no package publication or deployment is authorized.

The reusable workflow now checks out immutable policy 728da8e internally. Updating only a library caller's workflow reference would otherwise continue executing stale policy.

Validation:

• Actual Data PR133 source built as the version emitted by hosted CI (0.3.1.dev3+g8ca0c3828): base validator rejects its old version scope; fixed validator using the actual workflow-pinned policy object accepts it. The archived-source version override is explicit because the archive has no Git metadata.
• New built-wheel workflow regressions fail five cases against the old policy (5fail/4pass), including valid Data 0.3 versions and accidental Engineer use; corrected final scope tests pass. Further controls cover Diagnostic 0.2, both future version limits, wrong repositories, missing evidence and the Diagnostic expiry.
• Full suite 127 passed; 92.13% branch-aware coverage, above 90% gate. Ruff, format, ty, strict MkDocs, package build and all pre-commit hooks pass.
• Data's fresh October25 evidence: complete Databento resolver failure persists; core 66 tests pass. Diagnostic's fresh actual Python 3.15 dependency install/import probe passes; no full Diagnostic compatibility claim is inferred.

Refs #2; ml4t/data#37; ml4t/diagnostic#45. Those trackers remain open for actual compatibility completion. Library owners must adopt the merged immutable workflow revision and qualify their current candidates.

AI assistance: Codex reviewed the policy, prepared the configuration/decision/tests, executed the checks and drafted this description. Maintainer-directed scope and ordinary reviewed-merge authority were explicitly provided for this work.