fix: canonicalize theme preset slugs

[chubes4]

Summary

• canonicalize generated preset slugs before writing theme.json definitions and references
• apply the same WordPress-compatible contract to color, font-family, font-size, and spacing presets
• prove both deterministic projection and WordPress-emitted CSS variable resolution

Closes #1092.

Verification

• composer test
• php tests/contract/wordpress-site-plan.php
• git diff --check origin/trunk...HEAD
• built an SSI development package from SSI f0bbb5d and Blocks Engine 789ad9aa (SHA-256 4a943c58b196cc10822d3139680d8a4853b63761a0fe6ce04bd6e0363dc2ef1f)
• rematerialized the retained Cara Jane artifact on WordPress 7.1
• verified all eight routes at 1440px and 390px return HTTP 200
• verified every rendered body computes to Poppins, sans-serif
• verified WordPress emits and resolves the canonical font-family, font-size, and color preset variables

The preset fix improves affected typography, but this PR does not claim complete Cara Jane visual parity; unrelated layout differences remain.

AI assistance

OpenAI gpt-5.6-sol was used through OpenCode to inspect WordPress preset emission, implement and test the canonicalization contract, build the development package, and collect browser-computed runtime evidence. Chris Huber reviewed and is responsible for every line.