fix(linalg,mps): scale-safe truncation-error accumulation

[ultimatile]

Summary

The truncation-error paths accumulated sums of squares naively, so the reported error saturated to inf once a singular value's square exceeded the real type's range and underflowed to zero for the smallest discarded tails — the failure the shared scale_safe_norm / combine_norms kernel exists to avoid. This routes the truncation-error sites through that kernel.

Closes #480

Changes

• crates/ariadnetor-linalg/src/decomposition/mod.rs — trunc_svd_with_policy_dense: the final Frobenius-norm error via scale_safe_norm, and the target_trunc_err rank-selection scan folds the discarded norm with combine_norms, widening each value to f64 before it is squared.
• crates/ariadnetor-linalg/src/block_sparse_decomp.rs — cross_sector_truncate: the same two changes for the block-sparse twin.
• crates/ariadnetor-mps/src/truncate.rs — the four private step functions return the unsquared per-step error; the truncate_dense / truncate_bsp sweep drivers fold those errors with combine_norms.

Impact

The step functions are private and every call site is updated in the same diff. Public error semantics are unchanged — TruncResult.error and trunc_err remain the Frobenius norm of the discarded singular values — now finite and nonzero across the real type's range. The combine_norms-folded sqrt(sum of err^2) equals the previous accumulation mathematically.

Test plan

New tests pin the two failure modes and the scan fix:

• dense trunc_svd: f64 overflow stays finite (1e200), f32 underflow stays nonzero (1e-23), and the f32 rank-selection scan discards a value whose si * si overflows in the real type.
• block-sparse cross_sector_truncate: f64 overflow stays finite.
• dense sweep: an overflow-scale swept error stays finite.

Existing boundary tests (..._exact_boundary, ..._strict_boundary) and the reconstruction-identity pins are unchanged and still pass. cargo test (linalg + mps + core): 934 passed, 0 failed; clippy and fmt clean.

Notes

A direct block-sparse sweep overflow test is not included: the faer SVD backend fails to converge on the general block matrices a real chain presents at these magnitudes, so the per-step error the sweep would accumulate is never produced. That path's overflow safety is covered by composition — the block-sparse SVD-site overflow test supplies a scale-safe per-step error, and the dense sweep test exercises the identical combine_norms fold.

[Copilot]

Pull request overview

Routes truncation-error accumulation in both the linalg SVD truncation paths and the MPS sweep truncation paths through the shared scale-safe norm kernel (scale_safe_norm / combine_norms) to prevent overflow-to-inf and underflow-to-0 in extreme singular-value regimes, while preserving the public meaning of truncation error as the Frobenius norm of discarded singular values.

Changes:

• Replace naive sum-of-squares truncation-error accumulation with scale_safe_norm (tail norms) and combine_norms (incremental fold) in dense + block-sparse truncation code paths.
• Change MPS private per-step truncation helpers to return unsquared per-step error and fold sweep totals via combine_norms.
• Add regression tests covering f64 overflow scale, f32 underflow scale, and the f32 target-error scan overflow case; adjust existing test documentation accordingly.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
crates/ariadnetor-linalg/src/decomposition/mod.rs	Uses combine_norms for target-error scan and scale_safe_norm for final truncation error in dense truncated SVD.
crates/ariadnetor-linalg/src/block_sparse_decomp.rs	Mirrors the dense fix for cross-sector block-sparse truncation (scan + final error).
crates/ariadnetor-mps/src/truncate.rs	Switches sweep error aggregation to combine_norms and returns unsquared per-step errors from private truncation steps.
crates/ariadnetor-linalg/tests/decomposition.rs	Adds dense trunc-SVD regression tests for overflow/underflow and scan correctness.
crates/ariadnetor-linalg/src/block_sparse_decomp/tests/mutant.rs	Adds block-sparse trunc-SVD overflow-scale regression test.
crates/ariadnetor-mps/tests/mps/truncate.rs	Updates test commentary to match scale-safe sweep aggregation behavior.
crates/ariadnetor-mps/tests/mps/truncate_mutant.rs	Adds dense sweep overflow-scale regression test.
crates/ariadnetor-mps/tests/mps/truncate_block_sparse.rs	Updates documentation/comments to reflect combine_norms folding and adds note about infeasible bsp sweep overflow fixture.
.cargo/mutants.toml	Removes fragile line-number-based mutant excludes now obsoleted by the accumulation changes.

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