fix(semantic): resolve Option.insights against findings as well as prior_insights (closes #70)

[ksd3]

Summary

Closes #70. Option.insights now resolves against the union of prior_insights and findings at each scope, instead of only prior_insights. An option may cite either kind of Insight — external evidence motivating the choice (prior_insights) or a claim produced by this analysis that in turn justifies a downstream choice (findings).

This matches the semantics laid out by the @claude analysis on the issue thread: the constraint is inherently cross-collection, LinkML cannot express it, and the enforcement therefore lives in the semantic validator.

Change shape

_validate_option_insight_ref previously resolved bare ids against prior_insights only (and ../id-form refs against an ancestor's prior_insights only). Both lookups now union with the corresponding findings map:

• Bare id → {**prior_insights, **findings} at the node-local scope
• ../id, ../../id, ... → union of prior_insights ∪ findings at the resolved ancestor scope (via the existing _resolve_ancestor_scope walk)

findings is threaded through _validate_decisions from both call sites — root (validate_analysis) and sub-node (_validate_analysis_node). Scope-description strings in INVALID_INSIGHT_REF messages updated to say "prior_insights or findings" so author-facing errors stay accurate.

Companion PR

A small documentation / schema-description change lives in LightconeResearch/astra-spec#39 — clarifies Option.insights's expanded resolution in analysis.yaml and docs/specification.md. The two are independent (each merges cleanly on its own) but read better together.

Test plan

• pytest tests/test_validation.py — 83 passed (82 existing + 1 new)
• Full suite (minus the pre-existing test_init_generated_files_are_valid failure unrelated to this change, tracked separately): 213 passed, 21 skipped
• ruff check / ruff format --check / mypy clean on changed files
• New valid fixture tests/fixtures/valid/option_insights_reference_finding.yaml covers four cases: bare id → prior_insight, bare id → finding, ../id → ancestor prior_insight, ../id → ancestor finding
• Existing test_insight_ref_bare_id_does_not_walk_ancestors and test_insight_ref_escapes_too_far still pass — the union widens which map within a scope but does not widen which scope, so the cross-scope-isolation behavior is preserved