perf(frontend): keep editor/terminal stacks out of unrelated surfaces

[Harry19081]

Stacked on #829 (perf/frontend-bundle-and-leaks). Merge that first; this PR's diff is only the top commit.

Summary

Second frontend RAM pass: stop the CodeMirror / xterm / syntax-highlighter / chart stacks from being loaded by surfaces that never render them. Measured with a static import-graph walk per lazy chunk root (src/test/staticImportGraph.ts), before → after:

Surface (chunk root)	Before: heavy packages statically reachable	After
engines/ChatPanel/events/stream/agent-message (every chat message)	xterm, CodeMirror + langs, sql-formatter, react-syntax-highlighter, highlight.js, recharts, @a2ui, framer-motion, mammoth, jszip (1 747 files)	none
modules/MainApp/TeamInbox	xterm, CodeMirror, sql-formatter, framer-motion	none
modules/MainApp/Settings/SettingsSlot	CodeMirror, sql-formatter	none
modules/MainApp/AgentOrgs	CodeMirror, sql-formatter	none
modules/ProjectManager/{Projects,WorkItems,LinearProjects}	xterm, CodeMirror, sql-formatter, framer-motion	none
engines/Simulator/index	xterm, CodeMirror, sql-formatter, framer-motion	none
modules/WorkStation/shared/index.ts (barrel, ~80 importers)	xterm, CodeMirror, framer-motion	none

Production build: 565 → 583 chunks (new lazy boundaries), duplicated-module bytes 1.86 MB → 1.54 MB, total unchanged (code moves between chunks — the win is what a given surface loads into the heap).

Problem

• modules/WorkStation/shared/index.ts re-exported three heavy edges: GitFileDiffSplit (unreferenced dead code → all of features/CodeMirror), the SidebarModules block (whose module evaluation registers the Terminal tab sidebar → engines/TerminalCore → xterm + addons), and QuickActionsPanel (framer-motion). ~80 files import that barrel, so every workstation, PM, inbox and settings surface — and the agent-message renderer via Chat/Communication — statically reached the editor and terminal stacks.
• Several on-demand views were imported eagerly by their hosts: SimulatorMessages in agent-message (its siblings user-message/thinking already lazy it), the raw-transcript content in SessionRawTranscriptDialog (its sibling view already lazies it), A2UIRenderer (recharts/@a2ui) and ReactArtifactRunner (sucrase + embedded React runtime) inside CanvasPreviewSurface, SkillEditorPanel in Settings, CodeMirrorEditor inside MarkdownEditor, the canvas "source" tab viewer.
• Editor-only consumers imported the features/CodeMirror barrel, which also carries Diff, ConflictEditor, SqlEditor + sql-formatter.

Solution

• Remove the three heavy re-exports from the WorkStation shared barrel (with comments explaining why they must not come back); CodeEditor/index.tsx imports SidebarSlot from ../shared/SidebarModules — the same import that already carried the side-effect registrations, so tab-sidebar registration is unchanged. Delete the unreferenced GitFileDiffSplit.
• React.lazy + Suspense fallback={null} at the on-demand render sites listed above (all follow existing precedents in the same files/siblings).
• Deep-import @src/features/CodeMirror/Editor at the seven editor-only consumers.
• New guard src/app/root/__tests__/featureBoundaries.test.ts: nine surfaces asserted free of the editor/terminal/highlighter/chart stacks; failure output prints the import chain.

Potential risks

• Each lazied surface shows an empty pane for one local chunk load the first time it renders (simulator variant of an agent message, raw-transcript dialog, A2UI/React canvas kinds, skill editor, markdown edit tab, canvas source tab). Same trade the neighbouring code already makes; auto-dismiss/loading states are unaffected.
• Tab-sidebar registrations now depend solely on CodeEditor/index.tsx importing ../shared/SidebarModules (previously the barrel evaluated it as a side effect for everyone). CodeEditor is the only host that resolves SidebarSlot, so this is intended, but a future host in another module must import SidebarModules too — noted in the barrel comment.
• The boundary guard is regex-based (not a full resolver): it can miss exotic import forms, and it will fail on legitimate new heavy usage until the allow-list is updated with a reason.

Validation / Test plan

• pnpm typecheck clean; eslint clean on touched files (pre-commit lint-staged passed).
• Full pnpm test: 1110 files / 8720 tests green (includes the new boundary guard and the perf(frontend): break down the startup bundle and bound frontend leaks #829 guards).
• Production build succeeds; chunk-duplication analysis above.
• Manual: send/receive chat messages (chat + simulator variants), open the raw transcript dialog, view A2UI/HTML/React canvases, open Settings → Integrations → Skills editor and Policy rule wizard, Agent Orgs wizard, Canvas app source tab, Terminal/Source-Control/Benchmark tab sidebars in the code editor.