fix(fitstats): require Rust for public infit/outfit and ordinary M2

[seonghobae]

Summary

Public infit_outfit() and ordinary public m2() no longer silently fall back to Python/NumPy residual/reference arithmetic when the compiled core is absent or incomplete. Matches the S-X² / person-fit fail-closed ownership from #771.

Also retargets remaining S-X² / person-fit / infit / M2 NumPy-fallback coverage suites to the fail-closed contract.

Supersedes closed #774/#776 drafts and #775 (issue #627).

Evidence

• Local targeted ownership + cov suites: 43 passed
• Native core smoke: m2 and infit_outfit execute on compiled core

Product gates

Merge when python (aggregate + 3.12/3.14), rust, package, fuzz are green.

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📥 Commits

Reviewing files that changed from the base of the PR and between c91ae21 and 2946ffe.

📒 Files selected for processing (7)

• CHANGELOG.md
• docs/changelog.d/775-fitstats-infit-outfit-failclosed.md
• python/fast_mlsirm/fitstats.py
• tests/test_cov_c_fitstats.py
• tests/test_fitstats_infit_outfit_allocations.py
• tests/test_fitstats_infit_outfit_rust_ownership_failclosed.py
• tests/test_fitstats_m2_rust_ownership.py

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[seonghobae]

Closing as duplicate of #788, which carries the same public infit_outfit() / ordinary m2() fail-closed ownership on main plus documentation maturity vocabulary sync. Prefer the single canonical line to avoid competing product PRs.