docs: mark Phase 15 complete

[RNT56]

Summary

• mark P15-T13 and the full Phase 15 provider roadmap complete after PR test: add provider compatibility evals #105
• replace the stale planned web-result re-entry design with the shipped native drop-on-decode/client-fence boundary
• reconcile T08 ownership, T13 evidence, wave status, and T14 dependency ordering across the canonical task and architecture docs

Verification

• GOTOOLCHAIN=go1.25.12 make verify
• pinned golangci-lint run ./... (0 issues)
• git diff --check