Add GAM ts cohort attribution design spec

[aram356]

What

Adds the design spec for GAM attribution of Trusted Server A/B traffic: a page-level GPT key-value (ts=true) set via googletag.setConfig so Google Ad Manager reporting can segment impressions/clicks from pages delivered through Trusted Server against the unmodified production cohort.

Docs-only. No code changes, no implementation. Status: Design.

Scope of this PR

• Single new file: docs/superpowers/specs/2026-07-15-gam-ts-cohort-attribution-design.md.
• Branched fresh off main (excludes unrelated work that was sharing the original spec branch).
• Docs-format (Prettier) passes.

Open blocker — do not implement yet

The marker key name is not finalized. The spec provisionally uses ts, but the character-limit basis for rejecting the more descriptive trusted_server is unconfirmed (SOAP/REST document a 10-char key-name limit; other Help Center material implies 20; provisioning surfaces differ). This must be preflighted in the target GAM network before the contract is fixed:

• Tracked in GAM attribution for Trusted Server A/B traffic (page-delivery marker) #1027.

Follow-ups before an implementation plan is written

An external review raised additional design items to fold in once the key contract is settled: label GAM results as descriptive delivery attribution (not causal); make the inferred production partition executable (exact eligible units, identical A/B report filters, preflight zero-counts, 0 ≤ Report B ≤ Report A invariants); acknowledge monitoring cannot prove per-response completeness; specify rollback drain ordering; extend the collision audit to Prebid-generated slot targeting (pbjs.bidderSettings, adserverTargeting, setTargetingForGPTAsync); and preflight Enhanced key-value reporting availability/billing.

No implementation plan exists yet (docs/superpowers/plans/); it should be written against a main-rebased branch after #1027 resolves.