#22361: fix(macos/canvas): prevent A2UI reload loop and add A2UI MIME mapping

## Summary - Update macOS canvas auto-navigation logic so `shouldAutoNavigateToA2UI(lastAutoTarget:)` returns `false` when the current target already equals the last auto-navigation target, preventing immediate reload loops that wipe freshly rendered A2UI content. - Add extension-based MIME mapping for A2UI static files in `src/canvas-host/a2ui.ts` so `.js`, `.css`, `.json`, and `.map` always get browser-friendly content types before content-sniffing. - Add canvas-host test coverage for these MIME mappings in `src/canvas-host/server.test.ts`. ## Why this fixes the issue These changes stop redundant forced reloads from auto-navigation and ensure A2UI assets are served in executable formats that browsers can parse, so rendered content persists instead of flashing and disappearing. ## Tests - `pnpm vitest run src/canvas-host/server.test.ts` - `pnpm vitest run src/canvas-host/*.test.ts` ## Edge cases considered - A2UI HTML files continue to be served with `text/html; charset=utf-8` and live-reload script injection. - Unknown extensions still fall back to existing `detectMime(...)` behavior. - HEAD requests preserve the selected Content-Type header for each path. <!-- greptile_comment --> <h3>Greptile Summary</h3> Fixed A2UI reload loop and added MIME type mappings for static assets. The Swift change inverted the logic in `shouldAutoNavigateToA2UI` to prevent auto-navigation when the target hasn't changed (returning `false` instead of `true` when `trimmed == lastAuto`), stopping redundant reloads. The TypeScript changes added explicit MIME type mappings for `.js`, `.css`, `.json`, and `.map` files before falling back to content sniffing, ensuring browsers receive correct content types for A2UI assets. **Key changes:** - `CanvasWindowController.swift:369` - return `false` (was `true`) when current target equals last auto-nav target - `a2ui.ts` - added `A2UI_MIME_BY_EXT` lookup table with charset-utf-8 for common web file extensions - `a2ui.ts:208` - changed condition from exact match (`mime === "text/html"`) to prefix check (`mime.startsWith("text/html")`) to handle charset variants - `server.test.ts` - added comprehensive test coverage for all four MIME mappings <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The changes are well-scoped, logically correct, and include test coverage. The Swift logic fix correctly prevents reload loops by inverting a boolean return when targets match. The MIME type mappings follow web standards and have explicit fallbacks. The test validates all four new MIME mappings with proper cleanup. - No files require special attention _{Last reviewed commit: ad53db0} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->