Add N-tier cascade architecture (default N=2)

[doramirdor]

Summary

Adds support for configurable N-tier routing in NadirClaw. The router can now
operate as a 2-tier, 3-tier, 5-tier, 7-tier, or flat-arm cascade via YAML
config without code changes or classifier retraining.

The new default profile is N=2 (cheap + strong), based on empirical
RouterArena results showing N=2 beats N=3 on arena_F (0.7358 vs 0.7136).

What's new

• nadirclaw/tier_config/: YAML schema (Pydantic), hot-reload loader,
  tier selector, classifier score adapter.
• nadirclaw/tier_config/profiles/n2_default.yaml: 4-model cheap pool
  (gpt-4o-mini, qwen3-235b, deepseek-v3.2, claude-3-haiku) + 5-model strong
  pool (gpt-5-mini, deepseek-reasoner, deepseek-v4-flash, grok-4-1-fast,
  claude-sonnet-4). Verifier-gated cascade at tau=0.80, adjacent escalation.
• nadirclaw/tier_config/profiles/n3_legacy.yaml: 3-tier legacy back-compat
  for existing users.
• NTierCascade class: new entry point that consumes a tier profile. Legacy
  Cascade class untouched.
• wide_deep_asym_v3 classifier extended with a continuous score field
  (expectation over class probabilities), enabling any N value without
  retraining.

Why N=2

The 3-tier cascade's medium tier dilutes routing: medium-pool models
(claude-haiku, deepseek-v3.2, etc.) are roughly as expensive as the cheap
tier but less accurate on average. N=2 collapses the classifier's decision to
one boundary, lets the verifier handle the rest. RouterArena bake-off shows
N=3 at 0.7136 vs N=2 at 0.7358 on the official scorer.

Backward compatibility

• NADIRCLAW_TIERS_PROFILE unset (default) -> no auto-loading at import,
  legacy Cascade semantics preserved.
• 730 pre-existing tests pass unmodified.
• New optional score field on ClassificationResult is additive.

Tests

• 34 new tests in tests/test_tier_config.py covering schema validation,
  hot-reload, tier selection (cutoff + Pareto flat-arm), score adapter,
  env-var resolution, fail-closed fallback, backward-compat parity.
• Full suite: 764 passed, 0 failed.

Migration

Existing users: no action required. Default behavior unchanged.

To opt in to N=2:
```bash
export NADIRCLAW_TIERS_PROFILE=n2_default
```

Or to a custom YAML file:
```bash
export NADIRCLAW_TIERS_PROFILE=/path/to/your_profile.yaml
```

Related

• RouterArena submission using this architecture:
  Add Nadir router (verifier-gated cascade + cost-min baseline) RouteWorks/RouterArena#112
• Architecture spec: see project docs at https://getnadir.com

Contact: info@getnadir.com