[codex] Add signal definition enrichment

[bokelley]

Summary

Extends core/signal-definition.json with definition-side enrichment for signal meaning, methodology, modeling, jurisdiction applicability, consent basis, and per-signal data-subject-rights routing.

This keeps #5009's separation intact: product/buy-time composition stays on product signal targeting options, while stable signal semantics live on the published signal definition.

Key decisions

• Adds taxonomy as signal-definition metadata, not as a new signal-value-type.
• Keeps package targeting grammar limited to the existing binary, categorical, and numeric value types.
• Adds taxonomy.value_mappings so categorical allowed_values[] can map to stable taxonomy nodes without changing the wire value grammar.
• Declares parent taxonomy node behavior with taxonomy.parent_match_behavior so descendant expansion is explicit seller behavior, not an implicit schema rule.
• Uses channel-based data_subject_rights routing to avoid duplicating {url,email} blocks per right.
• Adds core/signal-modeling-disclosure.json instead of reusing creative provenance.disclosure, because modeled data-signal disclosure is not content provenance/render guidance.
• Tightens modeled-signal validation: modeled training jurisdictions must be non-empty, and required modeling disclosures must name applicable jurisdictions.
• Allows panel as a signal data_sources value for JIC/panel-derived audiences, while keeping co-viewing uplift and seller/JIC reconciliation on measurement/vendor-metric surfaces.
• Clarifies the broad-feed workflow: use get_signals for candidate discovery, then confirm product eligibility/composability through get_products.filters.signal_targeting or the product response before create_media_buy.
• Leaves selection_group and targeting-mode composition concerns out of signal-definition; those remain product/buy-time concerns.
• Rewords the signal docs and schema descriptions to use “published signal definitions” / adagents.json signals[], avoiding confusion with sync_catalogs.
• Addressed protocol and product expert review feedback for schema invariants, categorical taxonomy mapping, provider docs, get_signals/product workflow guidance, and JIC/panel measurement placement.

Validation

• npm run test:schemas
• npm run test:composed
• npm run test:schema-utf8
• npm run test:json-schema
• npm run test:storyboard-context-entity
• npm run check:x-entity-gaps (informational existing-gap report only)
• git diff --check
• precommit hook: test:unit, test:test-dynamic-imports, test:callapi-state-change, typecheck