Fix standalone poster HTML fidelity

[Happy (huangyangyu)]

Summary

• freeze selectable PowerPoint font families to bundled licensed DejaVu faces for deterministic browser metrics
• bake render-time expand into poster.html and validate the persisted file before PDF/PNG capture
• require accepted expand rules to survive a brand-new BrowserContext, including image decode and settled geometry
• guard legacy recurring fitters so data-role, poster-class, and body-only canvases measure unscaled geometry and preserve the viewer transform
• stop durable navigation at DOMContentLoaded before bounded settle so blocked non-layout subresources retain the soft render path
• freeze page scripts before applying the thumbnail-only transform, preventing late timer ticks from producing a live-only export
• keep HTML/PDF/PNG promotion transactional and document the durable fidelity contract

Runtime scope

• the paper2poster tests directory is intentionally excluded; the deployed skill does not need test fixtures at runtime

Validation

• /usr/bin/python3 -m py_compile on check_poster.py, render_poster.py, utils/font_fidelity.py, and utils/render.py
• paper2poster skill quick validation passed
• git diff --check
• real cold-browser canary on arXiv 2608.03571: reopened poster.html at 5760x3456, applied scale(0.35), waited 450 ms across recurring fitter ticks, and matched poster.png with 0 changed pixels; SHA-256 6018405841ce09502948b347c5cc0ef5b515e35caad21084f5725e5aa39b3817
• forced Trending backfill report 20260813T175752Z-421242-24341790: succeeded true, published 53
• post-publish HTTP verification: all 53 live reel.html responses matched the published SHA-256; both reported daily-paper URLs passed a fresh Chromium smoke with all relevant resources returning 200