chore(ci): bump Python to 3.14 and tighten conformance dep floors

[tymofiy]

Summary

Refreshes Python runtime and pinned versions for the conformance suite + markdown linting.

Thing	From	To	Rationale
python-version (CI)	3.12	3.14	Current stable; EOL 2030-10-31 (vs 3.12's 2028-10-31).
pyyaml (requirements.txt)	>=6.0	>=6.0.3	Current PyPI release; declares a tested baseline.
jsonschema (requirements.txt)	>=4.0	>=4.26	Current PyPI release; declares a tested baseline.
markdownlint-cli (CI install)	unpinned	@0.48.0	CI reproducibility — unpinned npm install -g installs whatever is latest at job time.

Why

1. Python 3.14 — 3.12 is fine but 3.14 gives a further EOL horizon (+2 years) with no compatibility cost: both conformance deps support Python 3.10+. Adopting the current stable line keeps the conformance surface on a supported runtime for the foreseeable future.
2. Dep floor tightening — >=6.0 / >=4.0 are extremely loose floors. Tightening them to the versions actually exercised by the conformance suite makes the declared contract honest. Floors (not pins) so upstream patches still flow.
3. Pin markdownlint-cli — a global npm install -g markdownlint-cli with no version selector means the CI job behavior drifts whenever upstream cuts a release. Pinning to @0.48.0 makes markdownlint rule coverage deterministic across runs; easy to bump in a future PR when desired.

Compatibility check

• pyyaml 6.0.3 — requires_python: >=3.8 ✓
• jsonschema 4.26.0 — requires_python: >=3.10 ✓
• markdownlint-cli 0.48.0 — engines: node>=20 ✓ (we use Node 24)

What this does NOT change

• Pinned GitHub Action SHAs (tracked separately in chore(ci): bump CI actions to versions running on Node 24 #10).
• Workflow structure, timeouts, permissions, job surface.
• node-version: "24" — already LTS.

Test plan

• CI green on GitHub Actions with Python 3.14
• Conformance suite still 12/12 on the new runtime
• markdownlint-cli@0.48.0 produces no new warnings on existing .md files

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Updates the CI/runtime baseline for the conformance suite by moving the workflow to Python 3.14 and tightening declared minimum versions for the suite’s Python dependencies, while also pinning the markdown linter version for more reproducible CI runs.

Changes:

• Bump GitHub Actions CI python-version from 3.12 to 3.14.
• Tighten conformance dependency minimums for pyyaml and jsonschema.
• Pin markdownlint-cli to 0.48.0 in CI.

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

File	Description
requirements.txt	Raises minimum supported versions for pyyaml and jsonschema used by the conformance runner.
.github/workflows/ci.yml	Updates CI to use Python 3.14 and pins markdownlint-cli for deterministic linting.

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