Materialize authored viewport metadata

[chubes4]

Summary

• validate authored viewport metadata shared by every imported route
• replace WordPress's frontend block-template viewport callback through the generated portable theme bootstrap
• preserve WordPress defaults in admin/editor and report unsafe, missing, duplicate, or conflicting declarations
• journal the materialization decision in import receipts and diagnostics

Closes #1495.

Verification

• npm test
• npm test -- --check
• php tests/smoke-viewport-metadata-materializer.php
• php tests/smoke-wordpress-site-plan-materializer.php
• homeboy review --placement local lint static-site-importer --path .
• WordPress 7.1 / PHP 8.4 canonical six-route import: one width=320, user-scalable=yes viewport tag per frontend route after SSI deactivation
• saved parse_blocks() result: 913 native blocks, zero freeform or responsive-layout fallback segments
• Gutenberg front-page probe: 235 blocks, zero invalid blocks, no content mutation, nested block selection succeeds
• all six routes: hidden navigation support label computes to display:none and 0x0
• desktop regression: 1280x2060, no overflow or browser errors, 0.0531% pixel delta from the accepted Loop 10 baseline

Paired acceptance build

• SSI: a72cb26892f65096178db6c296edd86f1c2bac0d
• Blocks Engine: 15cc71ed6b5bde3c4dbc89f295fdee8f0ef621d7 (Automattic/blocks-engine#1544)
• Package SHA-256: 5fbacb9deace10c196298214241241602b71c4596853b537322ca7c896263361

AI assistance

GPT-5.6 Sol through OpenCode traced the viewport handoff, implemented and tested the materialization contract, built the provenance-pinned package, and performed the WordPress import and browser acceptance checks under human direction.