Add MISSING_FILE_GENERATION special value for "file does not exist"

Implements a new special generation counter value (UINT64_MAX) that
allows clients to assert that a file should NOT exist when performing
operations.

Primary use case: Atomic "create-if-not-exists" operations
When combined with TOASTY_WRITE_CREATE_IF_MISSING flag:
- File doesn't exist → server creates file and writes (SUCCESS)
- File exists → gen_match fails → BADGEN error (prevents overwrite)

This enables race-condition-free file creation where you want to
create a NEW file or fail if it already exists.

Key changes:
- Added MISSING_FILE_GENERATION constant to file_tree.h
- Updated gen_match() to return false when checking existing entities
  against MISSING_FILE_GENERATION
- Modified file_tree_delete_entity() to succeed (no-op) when file is
  missing and MISSING_FILE_GENERATION is specified (idempotent delete)
- Modified file_tree_write() to validate against the new value
- Added documentation in metadata_server.c explaining how
  MISSING_FILE_GENERATION works WITH CREATE_IF_MISSING
- Updated protocol documentation in PROTOCOL.txt
- Added comprehensive feature documentation in MISSING_FILE_GENERATION.md

Additional use cases:
- Idempotent delete operations (delete only if not already gone)
- Detecting unexpected file creation in validation/testing scenarios

The implementation is backward compatible and type-safe. Files are
never assigned MISSING_FILE_GENERATION as their actual generation value.

misc/PROTOCOL.txt

@@ -13,6 +13,21 @@ All messages start with a shared header, defined as:
         uint32_t length;
     };
 
+1.1 Generation Counter Special Values
+
+Generation counters (uint64_t) have two special values:
+
+    NO_GENERATION (0):
+        When used in expect_gen, means "skip generation check entirely".
+        Files/directories are never assigned this generation value.
+
+    MISSING_FILE_GENERATION (UINT64_MAX):
+        When used in expect_gen, means "expect file/directory to NOT exist".
+        - For DELETE: succeeds if file doesn't exist (no-op)
+        - For WRITE: fails with BADGEN if file exists, fails with NOENT if missing
+        - For existing operations: causes generation mismatch if file exists
+        Files/directories are never assigned this generation value.
+
 2. Client Messages
 
 2.1 Client to Metadata Server messages