… fit() Union annotation + REGISTRY contract clarifications

Holistic codex re-audit of merged #402 (HAD Phase 5 agent surfaces) + #420 (cleanup) surfaced residuals that the per-PR CI review path could not see because the cleanup PR's diff scope hid the holistic state. 10 codex rounds against the combined post-PR state surfaced:

Methodology / contracts (REGISTRY):
- did_had_pretest_workflow Phase 3 Note still claimed the overall path "reduces a multi-period panel"; the validator actually requires exactly two periods. Updated to describe the aggregate-dispatched contract explicitly (overall = 2-period; event_study = multi-period).
- Phase 5 wave-1 notes still said the fit() return Union was "not test-enforced"; new regression closes the gap.

Code / API:
- HeterogeneousAdoptionDiD.fit() return annotation widened to Union[HeterogeneousAdoptionDiDResults, HeterogeneousAdoptionDiDEventStudyResults] so the static type contract matches the documented runtime polymorphism on aggregate. Updated the stale dispatch comment that claimed the annotation was single-period.
- ContinuousDiD -> HeterogeneousAdoptionDiD Step-4 handoff in practitioner.py now (a) explicitly recodes first_treat=inf -> 0 before both HAD example fits (HAD's _validate_had_panel rejects any first_treat outside {0, t_post}; ContinuousDiD's silent inf-normalization is HAD-incompatible), (b) frames the routing nudge around the WAS vs ATT(d) estimand difference rather than around the existence of untreated units, and (c) names HAD's stricter panel-shape and encoding requirements before showing the code.

Agent guides (llms-full.txt, llms-practitioner.txt):
- HeterogeneousAdoptionDiDEventStudyResults table now mirrors the single-period table's variance_formula and effective_dose_mean semantics (event-study path populates the same four pweight / survey_binder_tsl / pweight_2sls / survey_binder_tsl_2sls labels per had.py:639-648; effective_dose_mean inherits the same mass-point Wald-IV semantics per had.py:721-734).
- llms-practitioner.txt continuous-treatment decision tree rewritten estimand-first (WAS vs ATT(d)) with panel-shape contract spelled out for HAD (aggregate='overall' two-period vs 'event_study' multi-period; staggered last-cohort-only WAS warning).

Tests:
- tests/test_had.py::TestFitReturnAnnotation pins the Union return annotation via typing.get_type_hints — drift would have been silent before.
- tests/test_had_pretests.py::TestMultiPeriodWorkflow::test_overall_aggregate_rejects_multi_period locks the registry-described 2-period-only contract.
- tests/test_guides.py::TestLLMsFullHADCoverage::test_llms_full_had_event_study_mirrors_weighted_metadata_semantics asserts the event-study guide table covers all four variance_formula labels and the mass-point Wald-IV effective_dose_mean.
- tests/test_practitioner.py::TestHADDispatch::test_handle_continuous_step_4_recodes_first_treat_inf_for_had locks the inf->0 recode in the emitted snippet.
- Plus 5 additional regression tests for cross-surface alignment between the practitioner handler, llms-practitioner.txt routing, and the underlying estimator contracts.

CHANGELOG:
- Original #402 entry updated to credit the new return-annotation regression and clarify what the inspect.signature-based test does and does not pin.

No methodology changes, no behavior changes to fit() outputs on any existing surface — all changes are contract clarifications, guidance corrections, and test additions on surfaces that #402 + #420 already established.