Large files: bound highlighter cache, scan search once, row ranges as one edit

- HighlighterEngine caches spans only for rows near the requested row
  and trims the cache around it past 16k entries; per-row end states are
  still kept to resume lexing. Jumping to the end of a 78 MB C++ file
  cached every row's spans (854 MB RSS, now 197 MB), and the first edit
  near the top spent 270 ms freeing them.
- Plain and regex search read lines as views into the materialized
  buffer instead of Buffer::Rows(), which built (and kept) a string per
  line, O(lines x pieces) after edits. Plain search scans the whole text
  once and tracks rows by newline.
- transform_rows() applies a per-row rewrite of a row range as one edit
  (replace_rows_text). Indent/unindent region and visual-line insert,
  backspace, delete, newline and yank used one piece-table edit per row,
  each linear in the buffer: 1.1 s per keystroke with 5,000 rows selected
  in a 26 MB file (now 4 ms), and minutes to indent a large region.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015rcVQiiWyTzoNrsvLSaAfB

syntax/HighlighterEngine.cc

@@ -65,6 +65,7 @@ HighlighterEngine::GetLine(const Buffer &buf, int row, std::uint64_t buf_version
 	if (!stateful) {
 		hl_->HighlightLine(buf, row, result.spans);
 		cache_[row] = result;
+		trim_cache_locked(row);
 		return result;
 	}
 
@@ -86,14 +87,28 @@ HighlighterEngine::GetLine(const Buffer &buf, int row, std::uint64_t buf_version
 			states_[static_cast<std::size_t>(r)] = state;
 		if (r == row)
 			result = lh;
-		else
+		else if (row - r <= kCacheNear)
 			cache_.insert_or_assign(r, std::move(lh));
+		// Rows further above only needed their end state (kept in states_):
+		// caching their spans too held about 10x the file size in memory
+		// after jumping to the end of a large file.
 	}
 	cache_[row] = result;
+	trim_cache_locked(row);
 	return result;
 }
 
 
+void
+HighlighterEngine::trim_cache_locked(const int row) const
+{
+	if (cache_.size() <= kCacheMax)
+		return;
+	cache_.erase(cache_.begin(), cache_.lower_bound(row - kCacheNear));
+	cache_.erase(cache_.upper_bound(row + kCacheNear), cache_.end());
+}
+
+
 void
 HighlighterEngine::InvalidateFrom(int row)
 {