Relates to #3688, #4532, #3745.

Existing documents can open in reading mode per tab while retaining unsaved buffers, rendered content, selection and copying. Integrate Muya's reading mode with document state, typed IPC, buffered restore and source-view handoff.

Save and Save As share the existing save handler, including the main-process write barrier and saved-content acknowledgement. Locking waits for active writes and rejects queued or stale saves. Explicit close-and-save unlocks dirty documents; external changes retain dirty buffers and suspend automatic overwrites.

Entering reading from source closes the runtime source view through its normal unmount handoff. Unlocking stays in live editing; the startup source-mode preference is unchanged. Source tabs retain independent undo histories across tab switches and later manual source entry, including tabs with identical content. New and pending documents remain editable by default. Mode-switch failures restore the editor and report the failure even if rollback IPC also fails.

Validation:

• Desktop unit suite: 107 files / 1154 tests passed.
• Electron reading-mode and existing source undo/caret/save-marker regressions: 18 passed on macOS.
• The identical-content source-tab regression fails against the previous implementation and passes with the fix.
• ESLint, typecheck and desktop build passed.

Targets develop after merged #5799, #5800 and #5801. #5793 remains open as the downstream UI layer for switches, menus, shortcuts, tab indicators and shell localization. Reading-context and state notifications include all 12 desktop locales.