Add native reduction provenance snapshots

[xtraid]

Summary

• add an opt-in native ReductionExplanation sidecar with exact signal orders and builder-produced gadget spans while leaving the standard build allocation-free
• export closed, hash-bound reduction/manifest contracts and copy native provenance into immutable Python models
• render a square-only reduction view with formula, signal routing, boundary colors, gadget overlays, fixtures, golden PNG, tests, and documentation

Validation

• make check: 16 C binaries, 120 Python tests, Pages checks, native and cross-engine smoke
• renderer: 238 tests
• strict GCC and Clang, sanitizers, static analyzer, and Memcheck passed before rebase
• post-rebase range-diff is identical and the old/new trees are byte-identical
• post-rebase make check and renderer suite passed
• diff, JSON, secret, dependency, workflow, and artifact audits passed

Scope note

Cachegrind was not rerun locally because this packet does not modify the solver or its hot path; provenance is opt-in and the standard builder performs no provenance allocation. Sanitizers and Memcheck cover the new native ownership paths. The normal CI dynamic-analysis job remains unchanged.