Fix VS Code freezes on long Reasonix chats

[SivanCola]

Summary

• replace per-update full chat snapshots with revisioned, coalesced transcript patches
• stop hidden Webviews and editor selection changes from triggering unnecessary rendering
• bound transcript DOM rendering and persist only lightweight Webview UI state
• add long-history regression coverage

Root cause

Every editor selection change and every ACP streaming chunk sent the complete conversation to the Webview. The Webview then persisted the full snapshot, cleared the transcript, reparsed Markdown, and rebuilt every message node. Session cost therefore grew with both history size and chunk count until the renderer became unresponsive.

Impact

Long or restored Reasonix sessions now update incrementally. The Webview recovers from missed revisions with a full snapshot, pauses work while hidden, renders the latest 200 items by default, and lets users load older messages in bounded batches.

Fixes #7

Validation

• npm run lint
• npm test — 56 tests passed
• VS Code 1.131 Extension Host integration suite
• npm run package:vsix
• npm run verify:vsix-contents
• npm audit --omit=dev --audit-level=high