fix(web): address diff card review — split terminator, error arm, wire narrowing

- DiffBlock: an empty side contributes zero lines and a trailing newline is a
  terminator, so a create ending in a newline draws one added line (not a
  phantom empty one) and a full deletion draws no phantom + line.
- diffCardModel: narrow the wire diffs payload (card is the only validated
  field) so a malformed diff card falls back to the generic path instead of
  throwing inside DiffBlock.
- FileMutationRow: surface the result text when an errored mutation has no diff
  card, so a failed edit/write is more than a red dot.
- copyText ends its closed union on assertNever.
- Docs: drop the "bridge relativizes" claim, record the file-count divergence
  from the TUI footer, correct the built-boot overclaim, note why the row title
  outranks the view title, and make fixture turn 67 args self-consistent.
- Tests: terminator/empty-side/interior-blank rows, wire-narrowing null arms,
  the error-text arm and its name/code fallback, stopped state, no-path summary,
  and the registration/disposal shape.

packages/client/ui-primitives/tests/diff-block.spec.tsx

@@ -76,6 +76,26 @@ describe('DiffBlock structure', () => {
     const { container } = render(<DiffBlock diffs={[]} />)
     expect(container.firstChild).toBeNull()
   })
+
+  it('treats a trailing newline as a terminator, not an extra blank line', () => {
+    // A create whose newText ends in a newline is one added line, not two, and
+    // the footer counts one — the phantom `+ ` empty line the naive split drew.
+    const { container } = render(<DiffBlock diffs={[{ path: 'n.txt', oldText: null, newText: 'hello\n' }]} />)
+    expect(changeRows(container)).toEqual(['hello'])
+    expect(screen.getByText('└ +1 -0 · 1 file')).toBeTruthy()
+  })
+
+  it('renders a full deletion as removed-only with no phantom added line', () => {
+    // newText '' is zero added lines: an empty string must contribute nothing.
+    const { container } = render(<DiffBlock diffs={[{ path: 'gone.ts', oldText: 'a\nb', newText: '' }]} />)
+    expect(container.querySelectorAll('[class*="_add_"]').length).toBe(0)
+    expect(screen.getByText('└ +0 -2 · 1 file')).toBeTruthy()
+  })
+
+  it('keeps a genuine interior blank line', () => {
+    const { container } = render(<DiffBlock diffs={[{ path: 'a.ts', oldText: null, newText: 'x\n\ny' }]} />)
+    expect(container.querySelectorAll('[class*="_add_"]').length).toBe(3)
+  })
 })
 
 describe('DiffBlock footer', () => {