refactor(php-transformer): make font materialization plan-owned

[chubes4]

Summary

Continues #1360: canonical site plans own nonempty theme.font_materialization before identity calculation. Successful report and compact-view font fields project that canonical value, removing duplicate input derivation on the successful compilation path.

Shipped report/view fields remain compatible. Failed results without a plan retain their report-only handoff. Old no-font plans remain valid without the additive field. Font-bearing plan identities intentionally change because font intent now participates in approval identity.

This is the BE producer portion only. SSI consumer migration is deferred pending its checkpoint work and a coordinated producer dependency update. No release or dependency/version mutation is included.

Validation

• Reusable producer validation covers font declarations, stylesheet projections, typed webfont graphs, and canonical SVG asset/write linkage.
• Contracts cover stale identity rejection through the existing resolver, invalid stylesheet and receipt graph data, compact v1/v2 round trips, legacy no-font plans, successful projections, and failed no-plan output.
• Existing numeric weight acceptance is retained, including emitted weight 1200.
• Fresh candidate contract runner and WordPress-site-plan contract passed; full artifact parity passed all 305 fixtures.
• Font-bearing baseline/candidate comparison retained blocks, assets, plan assets, writes, and report font output. Additive plan font intent and its resulting identity change are intentional; whole-envelope equality is not claimed.
• PHP syntax and diff whitespace checks passed.
• All ten CI checks passed on 46fa2dc73fea185a57a92e543e2ca6961bbc2c20: Composer tests and WordPress integration across PHP 8.2-8.5, plus visual parity tools, and solved-site promotion.

Reproduce from php-transformer after installing Composer dependencies:

php tests/contract/run.php
php tests/contract/wordpress-site-plan.php
composer test:parity

AI Assistance

AI-assisted implementation and independent review through OpenCode, coordinated by GPT-6 Astra under Chris Huber's direction. Delegated agents implemented and reviewed the change, and fresh lab verification was required before publication.