feat: add last-good live preview

[tt-a1i]

Summary

• add an explicit desktop-only archify preview authoring loop on a random IPv4 loopback port
• bind every generation to an immutable JSON snapshot, reuse atomic verified delivery, recheck the live source digest immediately before commit, and preserve the last-good artifact through invalid, missing, failed, or superseded saves
• keep the preview shell separate from canonical HTML and exports; reject external hosts, write methods, arbitrary paths, and input/output aliases
• bound shutdown, terminate stuck delivery process groups, and remove server, timers, children, and private staging state
• document the opt-in Skill contract and same command across all README languages without changing the hero

Stability evidence

• npm test — 441/441 pass
• npm run test:webm — Share Card, Copy Share Card, all Route Cards, visual matrices, reduced motion, and WebM pass
• focused regressions cover missed watcher events, slow polling, A→B→A saves, symlink/future-path aliases, invalid/schema/check failures, burst de-duplication, latest-wins, bounded hung-child shutdown, and all five renderers
• rebuilt zero-dependency ZIP passes doctor, preview revision 1, artifact checking, clean Ctrl-C shutdown, closed-port verification, and zero staging residue
• in-app browser QA verified initial render, automatic valid refresh, invalid/schema last-good preservation, repaired refresh, diagnostic redaction, Style/Focus/Export interactions, no console errors, and no preview state in the output artifact
• two independent blocker/major review rounds reproduced and closed four race/shutdown/path defects; final review passed with no findings

Product boundary

Preview remains explicit and desktop-only. It is not started by default for one-shot generation, CI, unattended agents, remote sharing, or mobile use, and it adds no dependency, schema, renderer, layout, hosted service, storage, or canonical export state.

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