Preserve source nav anchor CSS on core navigation

[chubes4]

Summary

• Replays carried source CSS anchor rules against WordPress core/navigation wrapper markup.
• Preserves source subnav anchor styling when source selectors target direct nav anchors but WordPress renders ul/li/a wrappers.
• Adds focused coverage for base nav anchor rules and a:first-child selector mapping.

CV fixture proof

• Fixture: 31-personal-cv-academic
• Baseline aligned mismatch from prior proof: 5.04%
• Candidate aligned mismatch after this change: 2.70% (312662/11598080, raw full-page 3.04%, offset x=0, y=0)
• Editor validation: 219/219 valid, 0 invalid
• core/html: 0
• SSI matrix local proof run: 0c704eae-1779-4cf6-bb24-b01e10de13a0

Verification

• ./node_modules/.bin/vitest run src/__tests__/theme-source-assets-parity.test.ts
• ./node_modules/.bin/tsc --noEmit
• SSI one-fixture CV matrix: passed, visual gate disabled for measurement
• SSI four-fixture sanity matrix: passed for CV plus 8-local-service-business, 9-membership-community, 38-medical-clinic; run 65066956-c2a9-4db3-81da-3b47c989120c

Notes

• A Lab offload attempt failed before fixture execution because the SSI rig health check ran without hydrated Node dependencies on the runner (pngjs missing). The same documented matrix path passed locally with hydrated dependencies.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI GPT-5.5 via OpenCode
• Used for: Diagnosed the nav CSS selector drift, implemented the transformer/theme CSS compatibility change, added tests, and ran fixture verification.