fix(client): address PR #5 review — race condition, buffer corruption, input error

Critical fixes from Hoshe review:
1. Race condition: defer TCP connect to _process() with retry logic
   (MAX_CONNECT_RETRIES=20, 100ms interval) so server has time to bind.
2. Buffer corruption: disconnect on oversized message instead of
   clearing valid buffered data (_corrupt flag, fail-safe).
3. Silent input drop: send_input() returns Error so callers can detect
   encode/validation failures.

Warnings addressed:
- ServerProcess validates server_path exists before spawning
- SIGKILL and health check TODOs documented for future work
- Diagonal keybindings documented as intentional deferral
- send_message uses single put_data() call (no partial write risk)
- Static frame helpers documented as D-030 Layer 2 test-only

New tests (36 total, up from 33):
- Partial read scenario (chunked TCP delivery)
- Multi-message sequential decode (exercises buffer corruption fix)
- send_input error return on invalid action

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/protocol/server_process.gd

@@ -5,10 +5,14 @@ var _pid: int = -1
 
 
 ## Start the server process. Returns the PID, or -1 on failure.
+## Validates that server_path exists and is accessible before spawning.
 func start(server_path: String, args: Array = []) -> int:
 	if _pid > 0 and is_alive():
 		push_warning("ServerProcess: server already running (pid %d)" % _pid)
 		return _pid
+	if not FileAccess.file_exists(server_path):
+		push_error("ServerProcess: server binary not found at %s" % server_path)
+		return -1
 	_pid = OS.create_process(server_path, args)
 	if _pid <= 0:
 		push_error("ServerProcess: failed to start server at %s" % server_path)
@@ -16,6 +20,8 @@ func start(server_path: String, args: Array = []) -> int:
 
 
 ## Check if the server process is still running.
+## NOTE: OS.is_process_running() doesn't distinguish starting/healthy/zombie.
+## TODO: Add health check (e.g. TCP readiness probe) when server supports it.
 func is_alive() -> bool:
 	if _pid <= 0:
 		return false
@@ -23,6 +29,7 @@ func is_alive() -> bool:
 
 
 ## Stop the server process.
+## TODO: Send graceful shutdown signal before SIGKILL when server supports it.
 func stop() -> void:
 	if _pid > 0 and is_alive():
 		OS.kill(_pid)