fix(web): model erase-in-line, tab stops, and cross-line SGR

Six findings. Each terminal-semantics case was checked in a real terminal
(tmux, reading back the painted screen) before changing anything:

`100%\r\x1b[KOK` shows `OK`. Modelling the `\r` without its erase left the
previous frame's tail standing — a regression against the old truncate, since
`\r\x1b[K` is the single idiom every spinner and progress bar writes. Erase is
now part of the same replay, in all three parameter forms.

`a\tb\rXY` shows `XY      b`. Counting a tab as one column produced `XYb` and
destroyed the alignment this card exists to hold, so the cursor now advances
by terminal columns: tabs reach the next 8-column stop and a wide character
takes two cells.

`\x1b[31mabc\rX\nnext` paints BOTH lines red. A newline does not reset the
graphic state, so state threads from one replayed line to the next instead of
closing at each line end.

Only `m` accumulates into a cell's style now. Folding cursor and erase
sequences in grew the state string per redraw and emitted boundaries anser
had to discard.

The empty check reads the parsed lines the card renders rather than the raw
text: output that is only escapes or control bytes survives `trim()` yet
parses to nothing, and drew blank rows plus a copy control for invisible
bytes instead of the placeholder.

The gutter is the card's own left padding rather than a margin. Every render
site rewrites `margin` wholesale for its own indent, which silently cancelled
the reservation and let a container clip the dot.

The fixture sample no longer carries an `[exit code: 1]` line: the real bash
presenter consumes that marker precisely because the card shows the exit as
its own pill, so the built-bundle snapshot had pinned a frame showing it
twice — one the product path cannot produce.

packages/client/ui-primitives/src/TerminalBlock.tsx

@@ -154,7 +154,12 @@ export function TerminalBlock({
     const body = command.endsWith('\n') ? command.slice(0, -1) : command
     return body.split('\n')
   }, [command])
-  const empty = text.trim() === ''
+  // Read from the parsed lines the card actually renders, not from the raw text:
+  // output that is only escapes or control bytes (a lone reset, an OSC title, an
+  // erase) survives `text.trim()` yet parses to nothing visible. Judging it on
+  // the raw text drew an output box of blank rows plus a copy control for
+  // invisible bytes, and hid the placeholder that belongs there.
+  const empty = lines.every(line => line.every(span => span.text.trim() === ''))
   const hidden = lines.length - maxLines
   const capped = hidden > 0 && !expanded
   // Same split arithmetic as the TUI transcript's collapsed tool card, so a