chore: raise Python floor to >=3.12 (CQ-01 / CAL-02)

[ChelseaKR]

Bumps requires-python to >=3.12 ahead of Python 3.10 EOL (Oct 2026, STANDARDS dated obligation CAL-02) per CODE-QUALITY-STANDARD CQ-01. Aligns ruff target-version / mypy python_version / classifiers / CI matrix, and regenerates uv.lock for the new floor.

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[ChelseaKR]

BLOCKED — conflicts with accepted ADR 0001 and with the held v0.9.0 release scope; also incomplete as a floor change. Owner: @ChelseaKR (maintainer decision).

Why blocked (policy, not mechanics):

• docs/adr/0001-python-311-floor.md (accepted, backfilled 2026-07-09) states: requires-python = ">=3.11" stands; the floor rises only when a runtime dependency forces it or 3.11 leaves upstream support, and the change gets its own ADR and a minor-version release note. Neither trigger holds today: uv.lock on main still resolves for 3.11 (no dependency forces 3.12), and Python 3.11 security support runs to October 2027. The PR's cited driver — CAL-02 / Python 3.10 EOL (Oct 2026) — does not apply, because the current floor is already 3.11. ADR 0001 is the repo's declared, justified deviation from CODE-QUALITY-STANDARD CQ-01 ("a high floor costs exactly the adopters the tool exists for"), so merging this reverses a documented product decision without superseding it.
• Sequencing vs. the v0.9.0 hold: v0.9.0 release prep is intentionally held on upstream docs: clarify v2.1.0 conformance requirements MobilityData/transit-operational-data-standard#156 (still open as of 2026-07-31), with the v0.9 gate scope already closed by fix: close v0.9 conformance and release gates #69. Merging now folds a user-facing floor raise into that held release's Unreleased scope. If the raise is wanted, it sequences most cleanly as a deliberate post-v0.9.0 (v0.10.0) item.
• Incomplete even if approved: no superseding ADR; README.md:37 and CONTRIBUTING.md:10 still say "Requires Python 3.11 or newer"; no CHANGELOG entry for a user-facing support change (ADR 0001 explicitly requires the release note).

Mechanics are otherwise clean (verified today): MERGEABLE, merges clean into main via git merge-tree (branch is 1 docs-only commit behind — #78), all 13 checks green on the branch head 1061eb9 (test matrix correctly reduced to 3.12/3.13), .python-version on main is already 3.12, and Dockerfile/action.yml pin 3.13 so they are unaffected.

If you decide the raise should happen: add a superseding ADR (e.g. docs/adr/000X-python-312-floor.md marking 0001 superseded), update README/CONTRIBUTING/CHANGELOG, rebase, and this becomes merge-ready. I have not made those changes because they presuppose reversing ADR 0001, which is yours to call.