WIP: Compose interaction mode

[FuJacob]

Summary

Lands the foundation for Compose Mode: a new SuggestionInteractionMode that gathers broader AX-tree context and prepares a full draft for deliberate review, alongside the existing inline-autocomplete path. This PR is the Phase 1 + Phase 2 slice from rfc.md — mode plumbing, settings/UI surface, and the bounded AX tree walker — without the live Compose generation/typing flow.

Status: WIP draft. The plumbing is in place end-to-end (mode setting → snapshot → router → prompt → context collector → overlay preview hook), but Compose generation is not yet triggered by Tab and the manual draft-typing controller has not been wired into the coordinator. Autocomplete behavior is unchanged.

What landed in this branch:

• SuggestionInteractionMode enum, persisted in SuggestionSettingsModel and carried in SuggestionSettingsSnapshot.
• Menu bar Mode picker + Settings "Interaction Mode" picker with explanatory text.
• ComposeContextCollector — bounded ancestor walk + DFS over the focused AX subtree with role allow/block lists, node and depth budgets, and Task.yield() cancellation points.
• ComposeContextNormalizer, ComposePromptRenderer, ComposeTextNormalizer, ComposeRequestFactory — pure helpers in Cotabby/Support/.
• ComposeRequest / ComposeResult / ActiveComposeSession value types; ActiveGenerationSession sum type collapsing autocomplete + compose into one active slot.
• SuggestionGenerating.generateCompose(for:) added to the protocol; router forwards Compose to llama only and surfaces an unavailable state for Apple Intelligence + MLX.
• SuggestionOverlayControlling.showComposePreview + ComposePreviewView for a multiline preview surface distinct from inline ghost text.
• SuggestionInserter.typeDraft(_:shouldContinue:) for chunked synthetic Unicode typing with periodic cancellation checks.
• SuggestionAvailabilityEvaluator is mode-aware: Compose does not require Screen Recording for AX-only context.
• LlamaRuntimeManager.generateUncached so Compose's larger drafts cannot pollute the autocomplete KV cache.
• RuntimeModelCatalog.composeRequiredFilename + supportsCompose(filename:) for the planned tabby-depth-1 requirement.

What is not yet in this branch (deliberate next-PR scope):

• First Tab does not yet kick off Compose generation; second Tab does not yet trigger typeDraft. The coordinator currently disables predictions with "Compose Mode is selected. Draft generation will be enabled after the Compose request pipeline is installed." when the mode is set.
• Mode-aware "model required" UI (lock model picker to tabby-depth-1, surface download CTA when missing).
• Manual QA across GitHub PR comments, Gmail, Slack, Notes.

Validation

xcodebuild -project Cotabby.xcodeproj -scheme Cotabby -destination 'platform=macOS' build
# ** BUILD SUCCEEDED **

xcodebuild -project Cotabby.xcodeproj -scheme Cotabby -destination 'platform=macOS' build-for-testing
# ** TEST BUILD SUCCEEDED **

xcodebuild test-without-building -project Cotabby.xcodeproj -scheme Cotabby -destination 'platform=macOS'
# Executed 274 tests, with 0 failures

swiftlint lint --quiet
# 2 pre-existing cyclomatic_complexity warnings (SuggestionCoordinator+Prediction, ComposeContextCollector); no errors

New tests: ComposeContextNormalizerTests (4 cases) — whitespace, dedupe, navigation noise, and line/context bounds.

UI changes have not been exercised in a running build yet because Compose generation is not wired up; the Mode picker round-trips through UserDefaults but selecting Compose currently surfaces the "pipeline not installed" disabled reason.

Linked issues

Refs #66, #67, #68, #69, #70 — Compose Mode RFC issues.

Risk / rollout notes

• This PR was significantly stale (forked before the tabby/ → Cotabby/ rename, the MLX engine addition, the full-suggestion acceptance keybind, structured logging, multi-line completion, and several other features). The merge took two passes — 040e3ba resolved the initial conflict set; 071f9ea re-merged five additional main commits that landed mid-merge (most notably the multi-line completion toggle, which added isMultiLineEnabled to the snapshot). Reviewers should focus on:
  • SuggestionSettingsModel.snapshotPublisher — Combine chain restructured to fan in the new mode + timing + multi-line fields cleanly.
  • SuggestionEngineRouter.generateCompose — only routes to llama; Apple Intelligence and MLX throw unavailable rather than silently forwarding.
  • SuggestionAvailabilityEvaluator.disabledReason — Screen Recording is now gated behind interactionMode == .autocomplete.
  • SuggestionInteractionState.validateSessionForAcceptance — restricted to the autocomplete session because Compose has its own accept flow.
• userTags is a snapshot field reserved for Compose's prompt; it currently always emits [] until a tag editor lands. No persistence migration needed.
• pbxproj: one new manual test entry (ComposeContextNormalizerTests.swift). Source files auto-discover.
• Do not merge until the Phase 3+4 follow-up wires generation into the coordinator; merging this branch alone would land latent code with no visible effect.