feat(types): tighten Format.assets typing + emit named slot unions/Zod schemas

[bokelley]

Closes #1652.

Why

Issue #1652 flagged two related codegen gaps. After #1514 (the asset_type-discriminator fix) shipped, both gaps reduced to one root cause:

• tools.generated.ts redeclares the Individual*Asset slot types but doesn't import the per-asset-type *AssetRequirements interfaces from core.generated.ts, so the post-processor at scripts/generate-types.ts couldn't add requirements?: there. Format.assets[i] typed IndividualImageAsset without requirements, forcing consumers to cast to read it.
• The 12 Group*Asset types nested in RepeatableGroupAsset.assets[] were bare BaseGroupAsset aliases — no asset_type, no requirements. The same codegen bug, never patched.
• No named IndividualAssetSlot / GroupAssetSlot / FormatAssetSlot exports. The unions only existed inline in Format.assets[], so ts-to-zod produced no top-level union schemas. Consumers had to fork their own z.union([...]) or hand-roll a discriminated schema.

What

scripts/generate-types.ts::applyIndividualAssetDiscriminators now:

• Injects import type { ImageAssetRequirements, … } from './core.generated'; at the top of tools.generated.ts when those types aren't locally declared, so requirements?: references resolve. core.generated.ts is unaffected (already declares the types).
• Restores asset_type + requirements?: on the 12 Group*Asset shapes via a new GROUP_ASSET_DISCRIMINATORS table.
• Emits named IndividualAssetSlot, GroupAssetSlot, and FormatAssetSlot unions in both generated files.
• Replaces the inline anonymous unions in Format.assets[] and RepeatableGroupAsset.assets[] with the named types via regex. Both replacements count their hits and warn loudly on zero — silent no-op would otherwise leave Format.assets[] fallen back to the loose anonymous form without TS surfacing it.

ts-to-zod automatically picks up the new union exports, so IndividualAssetSlotSchema / GroupAssetSlotSchema / FormatAssetSlotSchema are now generated alongside per-type IndividualImageAssetSchema { asset_type, requirements } and GroupImageAssetSchema { asset_type, requirements }.

The hand-authored shim at src/lib/types/format-asset-slots.ts is reduced from ~200 lines of duplicated type system to a thin re-export that aliases the prior *Slot-suffixed names (e.g. `IndividualImageAssetSlot = IndividualImageAsset`) to the codegen names. Builders in format-asset-slot-builders.ts import codegen types directly. Two builders (briefGroupAsset, catalogGroupAsset) are removed — the spec doesn't include brief or catalog in RepeatableGroupAsset.assets[].oneOf, so calling these always produced wire-invalid output.

Compat

• Format.assets typing: strictly tightening. Adopters who relied on the loose shape (no requirements) get the field they were missing; the cast pattern in Codegen: Zod schemas for FormatAssetSlot, and tighten Format.assets typing #1652 goes away.
• *Slot alias optionality: the prior hand-authored shim modeled requirements as required; it's now optional (?:) to match the spec. Adopters that destructured slot.requirements and treated the value as defined will need to assert or handle undefined. Called out in the changeset.
• Removed builders: briefGroupAsset / catalogGroupAsset were always type-broken per spec. No FormatAsset.groupBrief / groupCatalog namespace entries existed (only the 12 valid ones are in the namespace). Internal grep finds zero callers across examples, skills, tests. Treating as a bug-fix removal under the minor bump.

Expert review

Three parallel reviewers (code-reviewer, ad-tech-protocol-expert, dx-expert) converged to ship-it with three minor follow-ups, all addressed in this PR:

• Broadened the requirementsLocallyDeclared sentinel regex to match interface | type (cheap forward-compat insurance).
• Added zero-replacement warnings on both assets[] rewrites so a future jsts layout change can't silently regress Format.assets[] to the anonymous union.
• Beefed up the changeset with the requirements-optionality migration note and the new *SlotSchema runtime-validation entry points.

Protocol verification: the 14-individual / 12-group split matches schemas/cache/latest/bundled/creative/list-creative-formats-response.json exactly. requirements is correctly optional (appears in properties but never in any branch's required:).

Test plan

• npm run typecheck — clean
• npm run format:check — clean
• npm run build:lib — clean
• Focused tests — 96/96 pass
  • test/lib/zod-schemas.test.js
  • test/lib/format-assets.test.js
  • test/lib/format-asset-slot-builders.test.js
  • test/lib/validation.test.js
  • test/examples/hello-creative-adapter-template.test.js
  • test/examples/hello-creative-adapter-ad-server.test.js
• Type-test (src/type-tests/format-asset-slots.type-test.ts) covered by root tsconfig — preserves the four PR chore: release package #118 invariants on the codegen types

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