[aliagga]

This PR closes two reproducibility/release gaps found in the current M3.2 state.

Changes:

• reuse the existing repository-wide reproducibility manifest in M3.2;
• persist an effective run configuration and run_manifest.json whenever the M3.2 gate runs;
• make check_m3_2.py persist its gate artifacts automatically;
• fix GitHub Actions so the grid dependencies required by the tests are installed;
• update README / M3.2 docs / changelog to reflect the actual milestone status.

No model, dataset, split, feature, or metric logic is changed.

Acceptance criteria:

• CI passes;
• M3.2 still ends in M3.2 GATE: GREEN;
• artifacts/gates/m3_2_lightgbm contains run_manifest.json, effective_run_config.json, validation_manifest.json, and model files.