Issue #66: vocab audit - single source of truth = 729 padded #522

Task

Audit vocabulary implementation to ensure single source of truth = 729 padded.

Acceptance Criteria

• vocab=729 (3⁶) everywhere (no 256 fallback)
• Padding: bytes 0-255 active, logits 256-728 = -inf
• No duplicate vocab definitions

Test

• Check all modules use vocab=729
• Verify padding implementation
• Run experiments/vocab/audit/

Refs

• Issue feat(spec): .tri Spec Linter — validate before pipeline #68: vocab=729 padded config

Commit

"fix(vocab): audit vocab=729 (single source of truth)"
Ref: #68