test(drift): redundant field_value_or_absent lint

[bokelley]

Summary

Follow-up to #876's review pass: warn when a storyboard asserts `field_value_or_absent` against a path the response schema already marks required. The tolerance is meaningless there — the spec guarantees the field is present, so the "absent" branch is dead code, and authors should use plain `field_value`.

Test-only change. No library code touched, no changeset.

Mechanism

• New helper `isPathRequired(schema, segments)` mirrors the existing `isPathReachable` in `test/lib/storyboard-drift.test.js`. Walks a Zod v4 schema along the parsed path; returns `false` when any segment traverses a tolerance wrapper (`optional`, `nullable`, `default`, `catch`), a `record` (no key guaranteed), or a union branch that omits the field. Conservative on unions (required iff EVERY branch requires it), aggressive on intersections (required if EITHER side requires it — same symmetry as `isPathReachable`).
• New describe block walks every storyboard's `field_value_or_absent` assertions and fails when the path is schema-required. Zero entries today (no storyboard uses the matcher yet); engages automatically as adoption grows.
• 10 unit tests pin `isPathRequired` semantics against inline Zod schemas — required string, optional, nullable, default, nested required object, intermediate optional wrapper, missing field, array element, record, union (both branches / only one).

Why test-only is the right surface

The review proposed a runner-time warning channel, but the runner has no cross-step warning surface today and nothing currently consumes one. Piggybacking on `storyboard-drift.test.js` is the right fit: it's already where storyboard paths meet Zod schemas, it already has an allowlist pattern (`UPSTREAM_SCHEMA_DRIFT`), and a CI test with an allowlist is a warning-shaped gate (fails the build, stays human-decidable). Adding a net-new runner warning channel + plumbing just for this check is speculative — revisit if other lint rules surface.

Test plan

• `node --test test/lib/storyboard-drift.test.js` — 399 pass / 4 skipped (pre-existing upstream drift)
• `npm run build` clean
• No storyboards break — the matcher isn't used in any existing storyboard yet (the spec-repo update at Document field_value_or_absent in storyboard-schema.yaml check enum adcp#3030 is the prerequisite for re-adding the `replayed` assertion that motivates it).
• CI green

Related

• feat(testing): field_value_or_absent check matcher (closes #873) #876 — `field_value_or_absent` matcher (the thing being linted)
• Document field_value_or_absent in storyboard-schema.yaml check enum adcp#3030 — spec-side enum docs (blocker for any storyboard using the matcher)
• Re-add replayed assertion on fresh create_media_buy using field_value_or_absent adcp#3031 — restore `replayed` assertion on fresh-path `create_media_buy` (first real user of the matcher)

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