fix: race conditions in file watcher, parser cache, and server state

[rosschurchill]

Problem

1. TOCTOU in file watcher — watcher.py checks file existence then reads it; file can be deleted between check and read
2. Unsynchronized parser cache — get_parser() in parser_phase.py has no locking; concurrent indexing threads can corrupt the cache or double-initialize parsers
3. Scattered global state — server.py uses separate _storage and _lock module-level globals with global statements, making mutation surface hard to reason about
4. No rate limiting — MCP server accepts unlimited requests with no throttling

Proposed fix

• RACE-1: Wrap read_file() call in try/except (FileNotFoundError, PermissionError, OSError)
• RACE-2: Add threading.Lock() with double-checked locking pattern in get_parser()
• RACE-3: Replace scattered globals with _ServerState dataclass
• AUTH-1: Add sliding-window rate limiter (200 requests / 60 seconds)

Files to change

• src/axon/core/ingestion/watcher.py
• src/axon/core/ingestion/parser_phase.py
• src/axon/mcp/server.py