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/connection/src/client/fixture.ts

@@ -238,7 +238,7 @@ function buildAlphaLog(): SessionEvent[] {
   // the presenter reads to emit the two-hunk sample: the card draws one path
   // header, the first hunk, a `⋯` gap, then the second (the same-file
   // second-hunk arm turns 62/63 cannot reach).
-  toolTurn(67, 'edit', '{"file_path":"src/config.ts","old_string":"multi","new_string":"multi"}', '已编辑')
+  toolTurn(67, 'edit', '{"file_path":"src/config.ts","old_string":"const timeout = 30","new_string":"const timeout = 60"}', '已编辑')
   // Turn 64: one run_code turn with three logged sub-dispatches — the Code
   // Mode acceptance surface (parent code row + nested native-identical rows,
   // including an isError sub-call and a bash sub-call that must hit the same