fix(equiv): Linux-canonical skip on non-Linux (#213)

[k-yoshimi]

Summary

Closes #213. Ends the macOS-only 1e-10 equivalence false-positive class without skipping for invisibility reasons. Equivalence tests are declared Linux-canonical: Ubuntu CI runner with gfortran 13.x is the authoritative environment; non-Linux platforms skip with an explicit, policy-documented reason.

What's in this PR (4 commits)

1. test(equiv): Linux-canonical skipUnless on all 7 modules — IS_LINUX = sys.platform.startswith("linux") + @unittest.skipUnless(IS_LINUX, ...) outermost decorator on TestEquivalence in eqlib/trlib/tilib/fplib/wrlib/wrxlib/totlib. Skip message references docs/baseline-policy.md.
2. docs: baseline-policy.md — Linux-canonical equivalence — new top-level policy doc (~140 lines) covering: what the 1e-10 contract asserts/does-not-assert, canonical platform, non-Linux behavior, local-verification via Linux container, prerequisites to promote a new platform to canonical.
3. docs(baseline-policy): link follow-up issue #215 — replaces gh-search placeholder with Python fixture parity for the 6-8 dead-baseline equivalence cases #215 link.
4. docs(baseline-policy): align predicate prose with code — Codex pre-push LOW fix: prose now says sys.platform.startswith("linux") matching the actual code.

Why Linux-canonical, not platform-keyed

The platform-keyed approach (committed as design history in docs/superpowers/specs/2026-05-26-platform-keyed-baselines-design.md, marked SUPERSEDED) was rejected after Codex execution-blocker review surfaced two structural prerequisites:

1. Graphics-stubs gap: 6 of 7 modules can't build standalone Fortran binaries on macOS (only tot has tot_static_stubs.f90). Memory reference_clavius_baseline_regen.md already documents this.
2. Python-fixture gap: 6-8 of 20 baselines lack <case>_params.py. Filed as Python fixture parity for the 6-8 dead-baseline equivalence cases #215.

Both gaps would need to close before per-platform baseline regen is feasible. Until then: Linux-canonical is the principled position.

Verification

macOS dev rig (Homebrew GCC 15.2.0):

$ pytest --forked --timeout=120 --timeout-method=signal \
    python/{eqlib,trlib,tilib,fplib,wrlib,wrxlib,totlib}/tests/test_equivalence.py
15 skipped in 0.72s

The 4 #213 failures (fp_dt1 RPCT, fp_iter01 RPCT, wrx_demo, wrx_iter01.pwr_tot) are gone — replaced with documented skips per the policy.

Linux CI (python-tests.yml:323 whole-tree pytest) continues to exercise all 7 equivalence suites on Ubuntu gfortran 13.x — IS_LINUX is True there, behavior unchanged.

Memory + index

feedback_equivalence_must_pass.md (controller-side) extended with a "Two skip classes" section distinguishing invisibility skip (FORBIDDEN, the existing rule) from principled platform-scoped skip (ALLOWED when canonical-env CI is documented). This PR is the reference example. Cargo-cult guardrail included.

Acceptance criteria (spec §8)

• All 7 test_equivalence.py have @skipUnless(IS_LINUX, ...).
• macOS pytest shows 15 skipped + 0 failures from these suites.
• docs/baseline-policy.md exists.
• Memory updated.
• Old platform-keyed spec + plan have SUPERSEDED banners (landed in F-2).
• Follow-up Python fixture parity for the 6-8 dead-baseline equivalence cases #215 filed.
• Both pre-push reviews HIGH/MED-free.
• Linux CI green (verifiable post-push).

Issue references

Closes #213.
Follow-up: #215.

Spec

docs/superpowers/specs/2026-05-26-linux-canonical-equiv-policy-design.md (Codex 3-round SHIP IT).

🤖 Generated with Claude Code

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Note

Low Risk
Test gating and documentation only; Linux CI still runs full equivalence; no production or auth paths change.

Overview
Linux-canonical equivalence replaces macOS 1e-10 baseline failures with an explicit, documented skip on non-Linux hosts.

All seven python/*/tests/test_equivalence.py modules now gate TestEquivalence with IS_LINUX = sys.platform.startswith("linux") and an outermost @unittest.skipUnless(IS_LINUX, …) that points reviewers to docs/baseline-policy.md. On Ubuntu CI (gfortran 13.x), behavior is unchanged; on macOS and other non-Linux dev machines, those suites skip instead of failing on libm/compiler drift.

The new docs/baseline-policy.md states what the 1e-10 contract does and does not cover, names Linux CI as canonical, explains WSL/Docker vs host OS, and documents how to verify locally and what must happen before promoting another platform (graphics stubs, Python fixtures, #215).

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[k-yoshimi]

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