Add TOASTY_WRITE_TRUNCATE_AFTER flag for HTTP PUT semantics

Adds support for truncating files after write operations, enabling proper
HTTP PUT semantics where the entire file content should be replaced.

Implementation:
- Added TOASTY_WRITE_TRUNCATE_AFTER flag (0x02) to ToastyFS API
- Updated file_tree_write() to accept truncate_after parameter:
  * When true, sets file size to exactly offset+length
  * Removes chunks beyond the new file size
  * Marks removed chunks for garbage collection
- Metadata server extracts and passes truncate flag to file_tree_write()
- WAL replay passes false for truncate_after (already handled in original write)
- Web proxy PUT now uses both CREATE_IF_MISSING and TRUNCATE_AFTER flags
- Updated documentation in PROTOCOL.txt, DESIGN.txt, and web/DESIGN.txt

Benefits:
- Proper HTTP PUT semantics (replace entire file content)
- Efficient: single write operation truncates and updates file
- Works with both sync and async APIs
- Garbage collection automatically removes orphaned chunks

inc/ToastyFS.h

@@ -58,6 +58,7 @@ typedef uint64_t ToastyVersionTag;
 
 // Write operation flags
 #define TOASTY_WRITE_CREATE_IF_MISSING (1 << 0)  // Create file if it doesn't exist
+#define TOASTY_WRITE_TRUNCATE_AFTER    (1 << 1)  // Truncate file after write offset+length
 
 // Creates a directory at the specified path.
 // Returns 0 on success, -1 on error.
@@ -129,9 +130,11 @@ int toasty_read(ToastyFS *toasty, ToastyString path, int off,
 //
 // For how vtag works, see toasty_read.
 //
-// The flags parameter can be set to TOASTY_WRITE_CREATE_IF_MISSING
-// to automatically create the file if it doesn't exist. A default
-// chunk size of 4096 bytes will be used for the created file.
+// The flags parameter accepts the following values:
+// - TOASTY_WRITE_CREATE_IF_MISSING: Automatically create the file if it
+//   doesn't exist. A default chunk size of 4096 bytes will be used.
+// - TOASTY_WRITE_TRUNCATE_AFTER: Truncate the file after offset+len,
+//   discarding any data beyond the write range. Useful for HTTP PUT semantics.
 int toasty_write(ToastyFS *toasty, ToastyString path, int off,
     void *src, int len, ToastyVersionTag *vtag, uint32_t flags);
 
@@ -185,9 +188,11 @@ ToastyHandle toasty_begin_read(ToastyFS *toasty, ToastyString path,
 // If vtag is not 0, the operation only succedes if the
 // tag matches the remote entity's.
 //
-// The flags parameter can be set to TOASTY_WRITE_CREATE_IF_MISSING
-// to automatically create the file if it doesn't exist. A default
-// chunk size of 4096 bytes will be used for the created file.
+// The flags parameter accepts the following values:
+// - TOASTY_WRITE_CREATE_IF_MISSING: Automatically create the file if it
+//   doesn't exist. A default chunk size of 4096 bytes will be used.
+// - TOASTY_WRITE_TRUNCATE_AFTER: Truncate the file after offset+len,
+//   discarding any data beyond the write range. Useful for HTTP PUT semantics.
 ToastyHandle toasty_begin_write(ToastyFS *toasty, ToastyString path,
     int off, void *src, int len, ToastyVersionTag vtag, uint32_t flags);