perf(renderer): diff code lines in O(n) via a prev-line map

[ly-wang19]

Problem

BaseCodeElement's line-diff useEffect runs on every [lines, animate] change — i.e. on each streaming update while a code block types out. It built a Set for the insert check (O(1)), then did a second pass with a linear prevLinesRef.current.find(...) per line for the content/replace check:

const prevIds = new Set(prevLinesRef.current.map((l) => l.id));
for (const line of lines) {
  if (!prevIds.has(line.id)) states.set(line.id, { type: "inserted", timestamp: 0 });
}
for (const line of lines) {
  const prev = prevLinesRef.current.find((p) => p.id === line.id);   // O(prevLines) per line
  if (prev && prev.content !== line.content) states.set(line.id, { type: "replaced", timestamp: 0 });
}

That second loop is O(lines × prevLines) = O(n²). Code blocks routinely run 50-300 lines and this fires repeatedly during the typing animation, so it's an on-render hot path.

Fix

One Map<id, prevLine>, one O(n) pass. Insert and replace were already mutually exclusive (a line either isn't in prev → inserted, or is in prev with changed content → replaced), so the merged if/else if is behavior-identical and also drops the redundant second iteration. Consistent with the Set the first loop already used.

const prevById = new Map(prevLinesRef.current.map((l) => [l.id, l]));
for (const line of lines) {
  const prev = prevById.get(line.id);
  if (!prev) states.set(line.id, { type: "inserted", timestamp: 0 });
  else if (prev.content !== line.content) states.set(line.id, { type: "replaced", timestamp: 0 });
}

Verification

• prettier clean, eslint clean, tsc --noEmit error count unchanged (the only errors are pre-existing unresolved-workspace-module noise; the edited region type-checks clean).
• Behavior preserved: identical states map for any (prev, current) line pair — typing on first render, inserted for new ids, replaced for same id + changed content, unchanged otherwise.

Closes #705