Consolidate tug-of-war tests into single DRY implementation

- Created test-tug-of-war.js with mode-based architecture:
  * basic: Simple P2P bidirectional frame exchange
  * variance: Codec comparison with warm JIT (from test-codec-variance-tug-of-war.js)
  * fuzz: Allocation-focused testing (placeholder)

- Extracted shared utilities into reusable module:
  * SeededRandom class for deterministic payload generation
  * generateFuzzedPayload() with full range fuzzing
  * sendFrames() with batching and fragmentation
  * TugServer chokepoint implementation

- Removed redundant files (DRY principle):
  * test-tug-of-war-server.js → integrated as --role=server
  * test-tug-of-war-client.js → integrated as --role=client
  * test-tug-of-war.sh → replaced by --mode=basic
  * test-p2p-fuzz.js → foundation for --mode=fuzz

- Preserved all functionality from removed files
- Matches prettier config (4-space indentation, 100 char width)
- Basic mode tested: 500 frames bidirectional, zero corruption

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