Why: I2 layering - between rules and the L2 classifier there is now
a deterministic host-scored semantic layer (rules then semantic then
model), and fallback can be rescued by a validated command
generator instead of going straight to the default channel.
Evidence: go vet clean; full suite ok, including ten new tests
(rerank resolve/below-threshold/tie/error-then-L2, generator
valid/invalid/error, catalog top-k).
Impact: IntentSpec gains Examples; Options gains Semantic (with
threshold), Generator; ambiguous candidates rerank through host
scores before L2; fallback validates generated commands
(action/slots/kind fail-closed) and synthesizes Top with slots;
generated commands are no longer overwritten by deriveCommands;
CatalogTopK by priority.
Refs: Documents proposal 2026-10-07-agent-stack-n1q1i1-plans.md I2
