Load into the piece table's original store; save and copy without materializing

A loaded file's bytes are adopted as the piece table's original storage
instead of being appended to the add buffer: the first keystroke no
longer reallocates and copies the whole file (140 ms at 200 MB), and the
read buffer is not copied again.

Save writes the pieces in order instead of materializing the buffer (a
second full copy that then stayed resident). Buffer::Bytes() assembles a
copy from the pieces and replaces four hand-rolled copies (swap
snapshots, recovery comparison, Command.cc's per-row join, the tests'
accessor). A single-piece table (an unedited file) is viewed in place;
GetLineView serves a line from its piece when it lies within one, so it
no longer copies the whole buffer after every edit. Code that needed the
whole text contiguous says so with ContentView().

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

Swap.cc

@@ -43,24 +43,6 @@ constexpr std::uint8_t MAGIC[8] = {'K', 'T', 'E', '_', 'S', 'W', 'P', '\0'};
 constexpr std::uint32_t VERSION = 2;
 
 
-static std::string
-snapshot_buffer_bytes(const Buffer &b)
-{
-	const std::size_t nrows = b.Nrows();
-	std::string out;
-	// Cheap lower bound: sum of row sizes.
-	std::size_t approx = 0;
-	for (std::size_t i = 0; i < nrows; i++)
-		approx += b.GetLineView(i).size();
-	out.reserve(approx);
-	for (std::size_t i = 0; i < nrows; i++) {
-		auto v = b.GetLineView(i);
-		out.append(v.data(), v.size());
-	}
-	return out;
-}
-
-
 static fs::path
 xdg_state_home()
 {
@@ -1313,7 +1295,7 @@ SwapManager::RecordCheckpoint(Buffer &buf, const bool urgent_flush)
 	p.buf          = &buf;
 	p.type         = SwapRecType::CHKPT;
 	p.urgent_flush = urgent_flush;
-	p.chkpt        = snapshot_buffer_bytes(buf);
+	p.chkpt        = buf.Bytes();
 	enqueue(std::move(p));
 }