feat(cli): results matrix — behavior × arm comparison table

[changliu2]

Adds assert-ai results matrix, a 2D comparison view for multi-run eval results. Companion to the bank-manager one-behavior-per-yaml configs in #262 — it renders the full N-failure-modes × M-arms grid that the viewer's single-suite compare view can't.

What changed

• New assert-ai results matrix SUITE/RUN ... under the results group.
• Repeatable --suite <SUITE> auto-expands all scored runs in a suite; pass several to span multiple behavior-suites in one table.
• --metric (default policy_violation), --json, --no-color, --results-dir.
• Rows = behaviors, read from each run's config.yaml behavior.name (falls back to manifest, then suite id).
• Columns = arm labels derived from run ids, ordered baseline → prompted → acs then any others alphabetically (deterministic).
• Cells = the metric rate; missing behavior/arm cells render - in tables and null in JSON.

Example

assert-ai results matrix `
  --suite bank-1b-distortion --suite bank-1b-data-leak `
  --suite bank-1b-policy-fab --suite bank-1b-unauth-txn `
  --metric policy_violation

Demo output (rendered from real runs)

       Behavior × arm matrix (Policy violation)
Behavior                      baseline  prompted  acs
no_financial_distortion       96.0%     88.0%     89.8%
no_sensitive_data_leak        53.3%     57.8%     45.5%
no_fabricated_policy          92.0%     84.0%     87.8%
no_unauthorized_transactions  60.0%     50.0%     26.0%

      Behavior × arm matrix (Strict overrefusal)
Behavior                      baseline  prompted  acs
no_financial_distortion       2.0%      0.0%      8.2%
no_sensitive_data_leak        2.2%      6.7%      2.3%
no_fabricated_policy          0.0%      0.0%      0.0%
no_unauthorized_transactions  2.0%      0.0%      2.0%

Validation

8 passed, 4 skipped, 1163 deselected