Step 4 of the discourse-planner sequencing. Replaces the contract-only
NotImplementedError with deterministic move-selection rules per
ResponseMode:
* BRIEF → 1 move (ANCHOR)
* EXPLAIN → up to 3 (ANCHOR + SUPPORT + RELATION)
* PARAGRAPH → up to 5 (ANCHOR + SUPPORT + RELATION + TRANSITION + CLOSURE)
* EXAMPLE → up to 3 (ANCHOR + RELATION + CLOSURE)
* WALKTHROUGH→ deferred, falls back to BRIEF shape so planner is total
Move selectors:
* ANCHOR — pack is_defined_as on intent.subject if available, else
first canonical pack fact on subject, else first
canonical fact of any source
* SUPPORT — pack belongs_to on anchor's subject
* RELATION — teaching/cross-pack chain rooted on anchor's subject
* TRANSITION — chain rooted on the relation's object (topic shifts)
* CLOSURE — no new fact; carries given lemmas forward
Empty bundles produce empty plans (planner is total — callers fall
through to the existing single-sentence composer path safely).
Updated contract test test_plan_discourse_is_contract_only ->
test_plan_discourse_handles_empty_bundle to reflect the implementation.
26 new behavior tests pin: per-mode shape (BRIEF/EXPLAIN/PARAGRAPH/
EXAMPLE/WALKTHROUGH), anchor preference for is_defined_as, support
preference for belongs_to, relation preference for teaching source,
paragraph transition topic shift, closure semantics (no new content,
carries given forward), fact uniqueness across moves, anchor fallback
when no pack subject match, and full determinism (byte-stable JSON
across all five modes, pure function equality).
Verification:
* 49/49 planner tests pass (23 contract + 26 behavior).
* smoke suite 67/67.
* cognition eval byte-identical:
public 100/100/91.7/100, holdout 100/100/83.3/100.
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