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/autoloads/input_mapper.gd

@@ -1,6 +1,8 @@
 extends Node
 
 # Semantic actions — NO raw key codes cross the bridge
+# Diagonal directions registered in project.godot with empty event arrays (intentional).
+# Keybindings deferred until input design is finalized — likely numpad or composite WASD.
 enum Action {
 	MOVE_NORTH, MOVE_NORTHEAST, MOVE_EAST, MOVE_SOUTHEAST,
 	MOVE_SOUTH, MOVE_SOUTHWEST, MOVE_WEST, MOVE_NORTHWEST,