Synthesize template parts only from separable chrome

[chubes4]

Summary

When the Blocks Engine plan emits no template parts, SSI synthesizes header/footer parts from source HTML landmarks. That fallback previously:

• accepted a nested <nav> as the global header purely via class-name signal, producing a nav-only header part while the page body kept its full embedded header (duplicate chrome)
• fell back to first_landmark_html, which grabbed any <header>/<footer> regardless of nesting — including chrome embedded inside <main> that the page body always renders itself
• scanned source files in array order, so an alphabetically-earlier archive page could define the global header instead of the entrypoint

Single-root documents (e.g. Figma-transformer output, where every landmark lives inside main.figma-root) therefore rendered chrome twice: once from the synthesized part and once from the page body — and the part could come from the wrong page variant entirely.

Changes

• A landmark is only extractable as shared chrome when it is separable — not a descendant of <main>. Applies to the document fragment decomposition (header, nav, footer) and the first_landmark_html fallback.
• Source files are scanned entrypoint-first so the imported front page is the authoritative source for synthesized parts; the transformer adapter now passes entry_path through to materializer artifacts.
• Classic static sites with body-level landmarks keep today's behavior (regression-tested).

Measured impact

Three-fixture production matrix (Fisiostetic, FSE Pilot Build Theme, Twenty Twenty-Five Community .fig transforms), front-page visual mismatch vs the served static source at threshold 0:

Fixture	Before	After
FSE Pilot Build Theme	45.55%	16.54%
Twenty Twenty-Five Community	42.22%	28.73%
Fisiostetic	32.15%	32.15% (unrelated cause, tracked separately)

Editor gate unchanged and fully green in the same runs: 727/727 native blocks, 0 core/html, 0 invalid, 365/365 browser-validated blocks.

How to test

1. php tests/smoke-embedded-chrome-template-parts.php — 17 assertions: embedded chrome produces no parts and stays in the page body; classic body-level chrome still produces parts; entrypoint page wins over alphabetically-earlier files; nested-chrome entrypoint falls through to the next separable page.
2. php tests/smoke-template-part-shell-dedupe.php
3. php tests/smoke-template-chrome-dedupe.php
4. php tests/smoke-theme-materializer-dry-run.php
5. php tests/smoke-semantic-parity-diagnostics.php
6. php tests/smoke-wordpress-site-plan-materializer.php

All pass locally.

AI disclosure

Implemented with OpenCode using openai/gpt-5.6-sol; all listed tests were run locally.