chore: release v0.12.1

[jayhesselberth]

Summary

Patch release bundling three bug fixes merged since v0.12.0:

• fix(train): drop selection_metric from --model-config before model construction #326 (fixes bug: optimize's best_params.json includes selection_metric, which train --model-config rejects #324) — leech model train --model-config no longer crashes on leech model optimize's own best_params.json output (selection_metric is now dropped before model construction)
• fix(merge): track chunk provenance instead of re-deriving group membership from labels #327 (fixes bug: data merge pairwise relabeling matches by original labels value, silently collapsing same-body contrasts to one class #323) — leech data merge pairwise relabeling now tracks chunk provenance by source file instead of matching on original label values, fixing a silent same-class collapse when two input files share an internal label
• fix(prepare): don't count reads absent from a pre-filtered POD5 as failures #328 (fixes bug: data prepare counts BAM reads absent from a pre-filtered POD5 as failures, aborting via MAX_FAILED_READ_FRACTION #325) — leech data prepare no longer misclassifies reads absent from a pre-filtered POD5 as failures, and the MAX_FAILED_READ_FRACTION check is now measured over reads actually attempted

Version bumped 0.12.0 → 0.12.1 in all three declared locations (pyproject.toml [project] version, pyproject.toml's leech-core== pin, rust/Cargo.toml), CHANGELOG.md assembled from changelog.d/ fragments via towncrier (added missing fragments for #326/#327; #325 already had one).

Test plan

• /check: ruff format --check, ruff check, snakefmt, ty check — all clean
• /docs: full documentation audit, zensical build — passes, no changes needed
• rust/build.sh rebuilt and reinstalled leech_core 0.12.1; check_rust() reports no version mismatch
• uv.lock/rust/Cargo.lock diffs are single-line version bumps only (no emscripten churn)
• CI (Lint / Rust lint+test / Test Python 3.12) — pending on this PR

After merge, the release is finalized by tagging main with v0.12.1 and pushing the tag, which triggers .github/workflows/release.yml to build wheels/sdists and publish to PyPI.

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