fix(acp): exhaustive result-card switch + tighten display-path guard

Address the render-intent-union review:

- toolResultUpdate branched on `if (card === 'terminal')` with a generic
  fallthrough; ToolResultView is a closed union, so make it an exhaustive
  `switch (view.card)` ending in assertNever (matching the call-side
  renderer and the § Conventions closed-union rule). Adding a result card
  later now fails to compile at the switch. Regression test: a rogue result
  card throws.
- displayTitle's `rel.startsWith('..')` guard mis-rejected an in-workspace
  target whose relative form merely begins with the chars `..` (e.g.
  `..cache/x`, a real sibling name), leaving its title absolute. Test for a
  `..` SEGMENT (`..` alone or `..<sep>…`) so such paths relativize, matching
  claude-agent-acp's `cwd + sep` prefix check. Regression test added.

packages/ui/acp/tests/stream-update.spec.ts

@@ -356,6 +356,26 @@ describe('ToolPresenter (tool-owned presentation via the tool registry)', () =>
     }))).toThrow('unreachable variant')
   })
 
+  it('an unknown render-intent RESULT card throws via the exhaustiveness guard (closed union)', () => {
+    // The result-side renderer is also an exhaustive switch + assertNever: a rogue
+    // result card (only reachable by a cast) must throw, so adding a real result
+    // variant later fails to compile at the switch.
+    const rogue: ToolDefinition = {
+      name: 'rogue',
+      description: 'r',
+      parameters: {},
+      execute: async () => [],
+      presentCall: () => ({ card: 'generic', title: 'r' }),
+      presentResult: () => ({ card: 'chart' }) as unknown as ReturnType<NonNullable<ToolDefinition['presentResult']>>,
+    }
+    const presenter = new ToolPresenter(registryOf(rogue))
+    expect(() => updatesWith(
+      presenter,
+      evt('tool/call', { turn: 1, step: 1, callId: CallId('c1'), name: 'rogue', arguments: '{}' }),
+      evt('tool/result', { turn: 1, step: 1, callId: CallId('c1'), content: [{ type: 'text', text: 'x' }], isError: false }),
+    )).toThrow('unreachable variant')
+  })
+
   it('forwards fs-tool render intents onto the wire (REAL read → generic locations, edit → diff content)', async () => {
     // Use the SHIPPING fs tools (not a stand-in), booted through their real
     // plugins, so the wire tool_call carries the actual presentCall output —
@@ -653,6 +673,17 @@ describe('relative-path display titles (bridge relativizes the title against the
     await ctx.fiber.dispose()
   })
 
+  it('an in-workspace file whose relative form starts with `..` chars (a sibling name) still relativizes', async () => {
+    // `/work/proj/..cache/x` is INSIDE the workspace — its relative form
+    // `..cache/x` begins with the chars `..` but is NOT a parent segment. The
+    // guard tests for a `..` SEGMENT, so this relativizes (matching the reference
+    // adapter, which accepts any target under `cwd + sep`).
+    const ctx = await fsCtx()
+    const update = callUpdate(ctx, '/work/proj', 'read', { file_path: '/work/proj/..cache/x.ts' })
+    expect((update as { title: string }).title).toBe('Read ..cache/x.ts')
+    await ctx.fiber.dispose()
+  })
+
   it('no session cwd → the absolute title is left unchanged', async () => {
     const ctx = await fsCtx()
     const update = callUpdate(ctx, undefined, 'read', { file_path: '/work/proj/src/a.ts' })