fix(web): bound the SGR state and follow real terminal widths

Nine findings, one critical. Terminal cases verified in a real terminal first.

CRITICAL: cells held the accumulated SGR history, so every state boundary
re-emitted the whole chain — output switching color without a full reset
emitted O(n^2) characters. Measured: 3200 such cells produced 25 MB, and the
reviewer's ~90 KB alternating-color case is well under bash's own output cap.
State is now a normalized record (foreground, background, attribute set) with
one canonical sequence per boundary, so the emitted text is linear in cells;
the 90 KB case parses in 36 ms. That also makes the attribute closers every
chalk-based tool writes actually close: `\x1b[1mbold\x1b[22mplain` leaves the
following write PLAIN, which a real terminal confirms.

Width follows emoji presentation, not the U+2600-U+27BF block: `A✓B` redrawn
with `XY` shows `XYB`, so the check every progress line writes is ONE column.
Taking the block as wide misaligned exactly the output this card exists for.

Writing over either half of a wide pair blanks the other, since a terminal
cannot leave one cell of a two-cell glyph standing.

`line\n\x1b[0m` does not end in a newline as a string yet its last parsed line
holds nothing visible, so the terminator check now reads the parsed lines — it
had added a blank row and inflated the collapse count.

A line with no cursor movement no longer builds a column buffer at all; only
its SGR is folded, so an `ls -R` or a 5k-line log allocates nothing per
character.

The `.terminalDescription` rule had been inserted into an existing grouped
selector, silently giving `.codeBody` description typography and changing its
bottom margin from 4px to 0 — a pre-existing surface this PR does not own.
Split out, `.codeBody`'s margin restored.

Three comments contradicted their code: the fixture's exit-marker line (still
claiming recovery from a marker deliberately removed), `bash-sample`'s header
(still routing a click to the details panel, and calling the consumer's cap the
block's own), and a DetailsPanel comment stacked above the wrong rule. The
ui-primitives README documented only the CR/BS half of the replay, so a reader
would expect `OK0%` where `100%\r\x1b[KOK` renders `OK`.

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

@@ -126,9 +126,19 @@ export function TerminalBlock({
 }: TerminalBlockProps) {
   const text = output ?? ''
   // A command's output ends with a newline; that terminator is not an extra
-  // blank line to draw or to count against the height cap. The copy control
-  // still copies `text` untouched.
-  const lines = useMemo(() => parseAnsiLines(text.endsWith('\n') ? text.slice(0, -1) : text), [text])
+  // blank line to draw or to count against the height cap. The check runs on the
+  // PARSED lines rather than on the raw text, because a reset after the final
+  // newline (`line\n\x1b[0m`) leaves the string not ending in one while still
+  // producing a last line with nothing visible in it. A genuinely blank final
+  // line — the double newline — survives, since it has a real empty line before
+  // the terminator. The copy control still copies `text` untouched.
+  const lines = useMemo(() => {
+    const parsed = parseAnsiLines(text)
+    const last = parsed[parsed.length - 1]
+    const terminated = parsed.length > 1 && last !== undefined
+      && last.every(span => span.text === '')
+    return terminated ? parsed.slice(0, -1) : parsed
+  }, [text])
   const [expanded, setExpanded] = useState(false)
   const [copied, setCopied] = useState(false)