refactor(linking): refresh fixed-anchor Rust ownership RED on current main

[seonghobae]

Controlled current-main replacement for #737 / issue #662

Fresh compare against protected main proved #737 is 30 protected-main commits behind and its only unique delta is tests/test_linking_fixed_anchor_rust_ownership.py. Its previous OpenCode handoff was explicitly bound to protected main 7b57f777..., so that write lease is stale. This Draft recreates the exact fail-first ownership contract on current protected main without transferring predecessor checks/reviews.

Intentional RED

The test replaces proposed _core.link_fixed_item_parameters with a sentinel linked result and requires public link_fixed_item_parameters() to return Rust-owned theta/alpha/b plus scale/shift evidence. Current Python-owned fixed-anchor affine arithmetic should therefore fail the delegation assertion after normal Rust/PyO3 package setup.

Exact identity

• protected base at creation: 8a56d7b1b024e18a5a96a94db4ede1bf6648ead1;
• clean replacement head: ac6c200fea7e0cb47a870e020faf753a61744b33;
• changed path: tests/test_linking_fixed_anchor_rust_ownership.py only.

Minimum GREEN

Move the exact current fixed-anchor scale/shift and linked theta/alpha/b transformation arithmetic into mlsirm-core; expose a thin _core.link_fixed_item_parameters PyO3 boundary. Python retains bounded validation, immutable marshalling and result/evidence construction only; missing compatible compiled capability fails closed. Preserve anchor/factor validation, dimensions without anchors, positive finite slopes, xi/zeta/tau, return types and caller immutability. Add direct Rust/public parity plus known-affine coefficient and parameter recovery using bias/MAE/RMSE, then complete docs/rustdoc/APA traceability and changelog after GREEN. Require one unchanged head with full Python 3.12/3.14, Rust/PyO3, package/reinstall, existing GPU/fuzz, Security/SAST and current-head governance gates.

Do not substitute another linking criterion, add GPU without benchmark evidence, change dependencies/workflows/version/release/hosted state, or create competing canonical docs. Advances #662. Supersedes #737 once established.

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[seonghobae]

Superseded by GREEN ownership PR (#753 form assembly / #755 linking). Closing RED test-only refresh.