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

PieceTable.h

@@ -4,7 +4,7 @@
  * PieceTable is kte's core text storage data structure. It provides efficient
  * insert/delete operations without copying the entire buffer by maintaining a
  * sequence of "pieces" that reference ranges in two underlying buffers:
- * - original_: Initial file content (currently unused, reserved for future)
+ * - original_: A loaded file's content (AdoptOriginal), never modified
  * - add_: All text added during editing
  *
  * Key advantages:
@@ -15,7 +15,8 @@
  *
  * Performance characteristics:
  * - Piece count grows with edit operations; automatic consolidation prevents unbounded growth
- * - Materialization (Data() call) is O(total_size) but cached until next edit
+ * - Materialization (Data() call) is O(total_size) but cached until next edit;
+ *   a single-piece table (an unedited file) is viewed in place, not copied
  * - Line index is lazily rebuilt on first line-based query after edits
  * - Range and Find operations use lightweight caches for repeated queries
  *
@@ -83,18 +84,39 @@ public:
 	// Content management
 	void Clear();
 
+	// Replace the content with `bytes`, taken over without copying (a loaded
+	// file). Undo spans into the old storage are invalidated, as by Clear().
+	void AdoptOriginal(std::string &&bytes);
+
 	// Accessors
-	char *Data()
+	// The whole text, contiguous; nullptr when empty. Valid until the next
+	// modification.
+	[[nodiscard]] const char *Data() const
 	{
-		materialize();
-		return materialized_.empty() ? nullptr : materialized_.data();
+		const std::string_view v = ContentView();
+		return v.empty() ? nullptr : v.data();
 	}
 
 
-	[[nodiscard]] const char *Data() const
+	// The whole text as one view: in place for a single piece, else the
+	// materialized copy (built once per modification). Valid until the next
+	// modification.
+	[[nodiscard]] std::string_view ContentView() const;
+
+	// A view of [byte_offset, byte_offset + len), clamped to the content: in
+	// place when the range lies within one piece (no copy), else into the
+	// materialized text. Valid until the next modification.
+	[[nodiscard]] std::string_view View(std::size_t byte_offset, std::size_t len) const;
+
+	// Call fn(const char *data, std::size_t len) for each piece, in order:
+	// the content without materializing it.
+	template<typename Fn>
+	void ForEachChunk(Fn &&fn) const
 	{
-		const_cast<PieceTable *>(this)->materialize();
-		return materialized_.empty() ? nullptr : materialized_.data();
+		for (const Piece &p: pieces_) {
+			if (p.len > 0)
+				fn((p.src == Source::Original ? original_ : add_).data() + p.start, p.len);
+		}
 	}
 
 
@@ -205,7 +227,7 @@ private:
 	void RebuildLineIndex() const;
 
 	// Underlying storages
-	std::string original_; // unused for builder use-case, but kept for API symmetry
+	std::string original_; // a loaded file's bytes (AdoptOriginal)
 	std::string add_;
 	std::vector<Piece> pieces_;