[codex] Unify package targeting resolution

[bokelley]

Closes #6132.

What changed

• Adds package placement selection under targeting_overlay, with selected and default modes.
• Introduces a unified per-package targeting_resolution for complete seller-applied targeting.
• Resolves properties, placements, and collections jointly, preserving each placement's effective property and collection scope.
• Moves demographic execution readback under targeting_resolution.demographics while preserving exact requested/applied predicates.
• Adds update_placements, PLACEMENT_SELECTION_INVALID, lifecycle documentation, compliance paths, and shared SDK vectors.

Why

Placement selection, property targeting, and demographic resolution previously used inconsistent package surfaces even though they are evaluated together. The unified model separates complete buyer intent from complete seller-applied state and requires sellers to reject invalid or non-equivalent intersections instead of silently dropping targeting.

Impact

Buyers send all targeting intent through package.targeting_overlay. Package responses return the requested overlay plus targeting_resolution; placement-targeted packages include resolved placements and the properties and collections where each can run.

Validation

• node tests/schema-validation.test.cjs
• node tests/composed-schema-validation.test.cjs
• node --test tests/placement-selection-schema.test.cjs
• node --test tests/demographic-targeting.test.cjs
• node --test tests/media-buy-targeting-overlay-vectors.test.cjs
• node scripts/build-schemas.cjs
• node scripts/build-compliance.cjs
• error-code, drift, oneOf, schema-link, docs-nav, changed-doc JSON, and snippet checks

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