Tighten V1 root API and fix mypy release gate

[MMerryweather]

Summary

• Selected .agents/plans/2026-05-31-fixes.md as the only active plan and executed it end-to-end on plan/2026-05-31-fixes.
• Finalized stable V1 root exports in rtdfeatures.__all__ and corrected downstream root-import usage to submodule imports for specialist kernels/learners.
• Aligned semver/API/docs contracts with stable root API, added docs contract coverage, and fixed remaining legacy test expectations.
• Cleared strict typing gate (mypy src tests) without weakening mypy config.
• Updated release gate artifact with passing evidence and removed stale waiver text.

Why This Plan Next

Only active plan outside done queue; directly unblocked release-gate readiness with minimal blast radius.

Phase/Commit Summary

1. 69014bb Finalize V1 root exports and fix downstream imports
2. e8381e2 Align semver/docs contracts and clear mypy for V1 API
3. 28c13e4 Update release-gate artifact and align remaining root API tests

Local Validation

• ruff check . -> PASS
• mypy src tests -> PASS
• pytest -m "not external_data" -v -> PASS (823 passed, 3 deselected, 2 xfailed)
• python -m build -> PASS
• twine check dist/* -> PASS
• pytest tests/test_semver_contract.py -v -> PASS (33 passed)
• pytest tests/test_public_api_docs_contract.py -v -> PASS (3 passed)
• pytest tests/test_sklearn_adapter.py -v -> PASS (26 passed)
• python examples/08_sklearn_adapter.py -> PASS

CI Required Checks

• Pending GitHub required checks on this PR branch. Merge is gated on green checks.

Independent Review Lanes (gpt-5.3-codex, medium)

1. Lane A: no findings; residual risk noted around full-gate reproducibility.
2. Lane B: no findings; targeted regression pass (116 passed) and low residual risks only.
3. Lane C: one low finding on pre-merge auditability (Commit: TBD until merge in release-gate doc), otherwise no correctness/scope issues.

Known Risks

• None blocking. Residual low-risk notes are documented in review lanes.

Retrospective

• Failure encountered: full pytest gate initially failed due three stale root-export expectations.
• Mitigation: delegated narrow test-contract fixes, re-ran full gate, then proceeded.
• Protocol updates applied this run:
  • Constrained sequential mode due unknown capacity visibility.
  • Native spawn-only delegation path enforced for all worker/review lanes.

Reusable Rules Enforced/Added

• Enforced: native subagent tooling only (no CLI delegation wrappers).
• Enforced: after actionable failure, run targeted remediation then rerun full gate before continuing.
• Added run-level note: if orchestrator rules file is missing, log tool_failure, continue with fallback checklist, and remain sequential.

Plan File Movement

• Local queue moved to .agents/plans/done/2026-05-31-fixes.md.
• Note: .agents/ is not tracked by git in this repository, so plan-file movement is reflected operationally but not as a tracked git diff.