[ONEGAYI]

When a Markdown comparison uses custom editors, the pencil command can reopen only one file instead of keeping the comparison. This change preserves the original/modified pair and toggles between a native text diff and the normally associated Markdown editors where supported by VS Code.

The native diff uses the real URIs, so the modified side remains editable and saves to the original file. Dirty source tabs are retained: closing one on VS Code 1.86 can otherwise revert an unsaved buffer even while a custom comparison displays it. Clean replaced tabs are closed to avoid accumulating comparisons. Opaque custom comparisons are resolved conservatively from their labels and open documents; ambiguous pairs are rejected rather than guessed.

A new Explorer action, Compare Selected Markdown as Text, accepts two Markdown files and opens a native text diff from the first view. Normal single-file associations are unchanged. The pencil menu now covers .markdown and uppercase Markdown extensions too. English and Chinese labels and usage instructions are included.

Validation

• npm run test:markdown: 24 tests passed on Node 20 and Node 24; no new dependency is required.
• Real extension-host integration: 9 scenarios passed on VS Code 1.86.2 and 1.138.0, covering first-open text tabs, both focus sides, closed source tabs, missing title URI, repeated switching, unsaved drafts, saving only the modified file, single-file switching, and concurrent switching in a second editor group.
• npm run build, changed-source ESLint and git diff --check passed.
• Two independent review-loop rounds completed; the .markdown pencil visibility issue found during review was fixed and rechecked.

Compatibility and limits

• Native forcing relies on the boolean override: true accepted by VS Code's vscode.diff converter, an option not present in the public typed API. Real-host regression tests document this compatibility seam.
• Recent VS Code versions may use the native text fallback even when reopening through normal Markdown associations; this PR does not override that platform behavior.
• Single-file switching is guarded for versions without the Tab API. The full VS Code 1.64 workbench and browser extension host were not run. Explorer command arguments and menu contributions are tested; visual right-click menu interaction remains a manual check.
• Integration test setup is documented in test/integration/README.md.

Based directly on upstream 908258d; this branch contains only the comparison fix and the Explorer entry, without fork-only changes.