Tasks

• [x] Add semantic_diff field to changed file entries in _spec_browser() payload
• [x] Add view-mode toggle (semantic / side-by-side / raw) to SpecsView component in dirty/refs modes
• [x] Render SpecDeltaView when semantic mode is selected for changed files
• [x] Keep "current" mode unchanged (no diff, just SpecContentView)
• [x] Test: dirty mode shows semantic diff for changed specs
• [x] Test: refs mode shows semantic diff for changed specs
• [x] Test: view-mode toggle persists across file selection within a session