[Qiyuanqiii]

Summary

Workspace Markdown files previously offered only numbered source. Add Source and Preview controls for .md, .markdown, and .mdown files, using the existing SafeMarkdown renderer and the already returned, redacted snapshot.

Source remains the initial view. Search hits and new citation locations return to numbered source. Switching views preserves the original file-reference payload, provenance, truncation indicator, and editing drafts. HTTP(S) links retain the existing new-window behavior; relative, local, and execution links remain inert, raw HTML is skipped, and remote images are omitted.

Stacked on #442. This PR targets Qiyuanqiii/workspace-search-hit-line so its diff contains only the Markdown preview change.

Validation

• CI passed for b6e89e8c481bb38293d584a9204b36916528c834: all 23 checks succeeded in workflow 39655433477.
• node node_modules/vitest/vitest.mjs run src/components/workspace-explorer-preview.test.tsx src/components/workspace-explorer.test.tsx src/components/safe-markdown.test.tsx src/components/file-edit-drafts.test.tsx — 38 tests passed.
• node node_modules/typescript/bin/tsc -b --pretty false
• node node_modules/vite/bin/vite.js build --logLevel warn
• Real Chrome journey against the production API and renderer: open Markdown, switch to Preview, search for a marker, and select the result. The preview returned to Source and highlighted line 24; the content digest stayed unchanged.
• git diff --check

Screenshots

Before, at #442 head 5892e371: Markdown can only be read as numbered source.

After: Preview shows formatted documentation and retains the Source control.

The committed acceptance README records the disposable sample and exact browser flow.

Surface governance

• No Surface is added, promoted, downgraded, deprecated, or removed.

N/A — no Surface change.

Audit

• No credentials or local runtime data are included.
• Policy, workspace, sandbox, and persistence boundaries were reviewed.
• The acceptance README documents the changed behavior and reproduction.