feat(core): add shared scale-safe norm accumulation primitive

[ultimatile]

Summary

The workspace carried four independent overflow-safe sum-of-squares accumulations: the dlassq-style scaled kernel behind ariadnetor-tensor's Frobenius norm, chained hypot in the incremental QR's R^-1 row norms, and two more chained-hypot accumulations in the SRC apply path (the leave-one-out estimator and the sketch-norm accumulation). This PR extracts one scale-safe kernel into ariadnetor-core and migrates all four sites onto it, unifying the NaN contract on NaN propagation. Closes #478.

Changes

• crates/ariadnetor-core/src/norm.rs (new): the dlassq-style scaled accumulator, kept private behind two public functions — scale_safe_norm(impl IntoIterator<Item = R>) and combine_norms(a, b) for combining two finished norms.
• crates/ariadnetor-tensor/src/norm.rs: frobenius_norm becomes a thin wrapper mapping elements to their magnitudes; kernel-level tests move to core, the complex-modulus mapping test stays.
• crates/ariadnetor-linalg/src/incremental_qr.rs: row_norm and the Pythagorean combine of grown row norms route through the kernel.
• crates/ariadnetor-mps/src/apply/successive_randomized.rs: leave_one_out_estimate and the sketch-norm accumulation route through the kernel; the positivity debug_assert moves ahead of the norm call so its coverage does not depend on how the kernel consumes the iterator.
• Test assertions on relative error unify on approx::assert_relative_eq! (approx was already a workspace dependency; added to core and tensor dev-dependencies).

Impact

• frobenius_norm consumers (dense / block-sparse storage): unchanged signatures; the kernel is the same algorithm relocated, so results are identical.
• The SRC adaptive loop and r_inverse_row_norms consumers see rounding-level perturbation only; every observing test compares with relative tolerances.
• Intentional behavior change on non-finite data: the hypot-chain sites returned inf when NaN and inf both entered an accumulation (IEEE hypot(inf, NaN) = inf); they now propagate NaN, matching frobenius_norm, and the core tests pin both orderings. On such (already poisoned) inputs the adaptive loop now grows to its bond cap instead of breaking early — bounded either way; Surface non-finite sketch state in SRC apply as an error #481 tracks surfacing the poisoned state as an error instead.

Test plan

• Kernel tests in core: 3-4-5 exactness, empty/zero input, f32/f64 extreme magnitudes, reciprocal extremes, NaN alongside infinity in both orders, infinities, and combine_norms.
• Migration safety net: inverse_row_norms_survive_extreme_scales (incremental QR) and src_adaptive_is_scale_invariant (SRC) — the two tests that would catch an unscaled replacement kernel — pass.
• cargo make gate (fmt-check, clippy with denied warnings on all targets, workspace tests) passes.

Notes

• The truncation-error pipeline (trunc_svd, its block-sparse twin, and the mps/truncate.rs sweep sums) still accumulates squared singular values / errors naively; Migrate truncation-error accumulations onto the scale-safe kernel #480 tracks migrating it — the squares are embedded in the rank-selection logic, so it is not a call swap.
• SRC apply still returns a poisoned state as a success on non-finite inputs; Surface non-finite sketch state in SRC apply as an error #481 tracks surfacing it as an error, following the NonFinite error variant the Lanczos driver in ariadnetor-algorithms already uses.

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[Copilot]

Pull request overview

This PR extracts a shared, scale-safe sum-of-squares accumulation kernel into ariadnetor-core and migrates existing norm-like accumulations across the workspace to use it, unifying overflow behavior and NaN propagation semantics.

Changes:

• Added ariadnetor_core::scale_safe_norm and ariadnetor_core::combine_norms as the shared scale-safe accumulation primitives (with pinned NaN/∞ behavior via tests).
• Refactored Frobenius/row-norm and SRC-related accumulations to route through the shared kernel (ariadnetor-tensor, ariadnetor-linalg, ariadnetor-mps).
• Standardized relative-error assertions onto approx::assert_relative_eq! by adding approx as a dev-dependency where needed.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
crates/ariadnetor-core/src/norm.rs	Introduces the shared scale-safe accumulation kernel + tests, including NaN/∞ contract.
crates/ariadnetor-core/src/lib.rs	Wires the new norm module and re-exports scale_safe_norm / combine_norms.
crates/ariadnetor-core/Cargo.toml	Adds approx for core’s kernel tests.
crates/ariadnetor-tensor/src/norm.rs	Replaces the local scaled kernel with a thin wrapper over scale_safe_norm (magnitude mapping).
crates/ariadnetor-tensor/Cargo.toml	Adds approx for updated test assertions.
crates/ariadnetor-linalg/src/incremental_qr.rs	Migrates row-norm accumulation and norm-combination to scale_safe_norm / combine_norms.
crates/ariadnetor-mps/src/apply/successive_randomized.rs	Migrates SRC reciprocal and sketch-norm accumulations to the shared kernel; moves positivity debug_assert ahead of iteration.
Cargo.lock	Records the new dev-dependency edges for approx.

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