docs: reorganize TODO.md into priority tiers

Restructure the backlog from a flat list into three tiers:
- Tier 1: compiler correctness bugs (schema mismatches, fall, codegen fallbacks)
- Tier 2: core language completion (slice fields, C backend parity, stdlib, unimplemented spec features)
- Tier 3: big bets needing design decisions (tensors/AI, lifetime model, GPU)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: code5717