SMITH-016 Implement editor groups, tabs, diff and merge foundations

[webfish0]

Outcome

Developers can manage multiple files and compare changes.

Scope

• Groups, splits, tabs, preview/pinned state, and history.
• Side-by-side and inline diff presentation.
• Dirty/conflict indicators and close confirmation.
• Initial textual merge workflow and follow-up compatibility boundary.

Acceptance criteria

• Workbench golden and E2E tests cover split, resize, diff, conflict, and close flows.

Requirements: FR-009, FR-010, US-002, US-003

Source: backlog.md SMITH-016.

[webfish0]

MVP-3 remote editing/workbench model landed and was verified on clean commit 763fad6.

Evidence summary:

• Command: npm run verify:mvp
• Result: passed
• T-004: passed — workspace file service supports read/write/list/rename/delete/search
• T-006: passed — Unicode helpers distinguish UTF-16 positions, graphemes, and display cells
• T-007: passed — text buffer supports edit, undo, redo, save, and stale-version conflict detection
• T-013-subset: passed — Explorer, Search, Problems, and Output view models are usable
• Evidence path from local/CI run: test-evidence/mvp-3-remote-editing/

Known limitation: this slice proves the editing and workbench models against fixture workspaces. Wiring the same model to live Code OSS remote filesystem/watch channels remains pending before closing the full remote editing milestone.