Define canonical webfont materialization proof contract

[chubes4]

Summary

• make blocks-engine/webfont-materialization/v1 the canonical imported-webfont contract
• emit deduplicated import, face, receipt, source, axis, range, unicode, digest, diagnostic, and browser-readiness records
• preserve explicit legacy projections for mixed-version consumers
• deduplicate repeated fixture stylesheet references by canonical source/hash/href
• prove fixture 37 yields one Inter import with nine faces and nine required receipts

Verification

• composer test
• php tests/contract/run.php
• php tests/parity/run.php (252 fixtures)
• PHP lint and git diff --check

Compatibility and rollout

Legacy plan fields remain as a marked compatibility projection. SSI #742 consumes the nested canonical contract but must wait for this change to merge and ship in the next PHP Transformer patch release, followed by an SSI dependency bump.

AI assistance

OpenAI GPT-5.6 Sol via OpenCode designed the contract, repaired cross-repository schema mismatches, and added actual fixture-37 deduplication proof. Chris Huber reviewed and remains responsible for every line.

Fixes #708.
Related: Automattic/static-site-importer#742.