Add GAM cohort attribution for Trusted Server traffic

[aram356]

Summary

• Add a default-off gam_attribution_enabled GPT option that applies fixed page-level ts=true targeting to eligible publisher documents whose rewritten head is emitted by Trusted Server, allowing GAM to describe treatment delivery while the production/control path remains unmarked.
• Apply targeting before publisher GPT initialization through the early bootstrap, with an exact document.currentScript attribute-gated TSJS fallback that fails closed and isolates setConfig failures.
• Preserve auction behavior, consent, the slot-level ts_initial lifecycle, and publisher/Prebid targeting while documenting the collision audit, reporting contract, rollout, monitoring limits, and drain-safe rollback sequence.

Changes

File	Change
crates/trusted-server-cli/tests/config_env_overlay.rs	Verify the GAM attribution environment override is applied when the corresponding TOML leaf exists.
crates/trusted-server-core/src/creative_opportunities.rs	Characterize that operator-provided slot-level ts targeting remains forwarded unchanged.
crates/trusted-server-core/src/html_processor.rs	Add integration-provided attributes to publisher TSJS tags and cover enabled, disabled, and source-order behavior.
crates/trusted-server-core/src/integrations/gpt.rs	Add the default-off setting and drive both inline activation and fallback authorization from the parsed GPT configuration.
crates/trusted-server-core/src/integrations/gpt_bootstrap.js	Queue page-level ts=true before the existing adInit guard and isolate attribution failures from later GPT commands.
crates/trusted-server-core/src/integrations/registry.rs	Add and test the integration hook that supplies trusted static attributes for publisher bundle tags.
crates/trusted-server-core/src/publisher.rs	Cover streaming-head attribution semantics and operator-targeting preservation.
crates/trusted-server-core/src/tsjs.rs	Render trusted static attributes on publisher TSJS tags without changing generic or creative tag output.
crates/trusted-server-integration-tests/browser/tests/shared/script-injection.spec.ts	Verify attribution metadata is absent from the default-off integration fixture.
crates/trusted-server-integration-tests/fixtures/configs/trusted-server.integration.toml	Keep the new configuration leaf explicit and disabled in integration tests.
crates/trusted-server-js/lib/src/integrations/gpt/index.ts	Add the executing-script-gated bundle fallback for page-level GAM targeting.
crates/trusted-server-js/lib/test/integrations/gpt/ad_init.test.ts	Verify SPA cleanup does not clear the page-level ts key.
crates/trusted-server-js/lib/test/integrations/gpt/gpt_bootstrap.test.ts	Exercise activation ordering, disabled behavior, unavailable or throwing setConfig, and bootstrap continuity.
crates/trusted-server-js/lib/test/integrations/gpt/index.test.ts	Cover exact fallback authorization, fail-closed cases, duplicate-ID decoys, cloned tags, and error isolation.
crates/trusted-server-js/lib/test/integrations/prebid/index.test.ts	Characterize publisher and Prebid-generated ts targeting across refreshes.
docs/guide/integrations/gpt.md	Document configuration, environment-overlay requirements, attribution semantics, collision checks, reporting, rollout, and rollback.
docs/superpowers/plans/2026-07-15-gam-ts-cohort-attribution.md	Add the reviewed implementation and verification plan.
docs/superpowers/specs/2026-07-15-gam-ts-cohort-attribution-design.md	Finalize the fixed marker contract, streaming meaning, reporting scope, and operational safeguards.
trusted-server.example.toml	Add the explicit default-off setting and environment-overlay guidance.

Launch gates

This PR completes the repository implementation. Starting the cohort remains gated on target-network GAM provisioning, cross-system collision and privacy/CSP audits, a paired-report dry run, and router-controlled treatment validation described in the design.

GAM results are descriptive delivery attribution, not causal treatment-effect estimates.

Closes

Closes #1027

Test plan

• cargo test-fastly && cargo test-axum
• cargo clippy-fastly && cargo clippy-axum
• cargo fmt --all -- --check
• JS tests: cd crates/trusted-server-js/lib && npx vitest run (44 files, 847 tests)
• JS format: cd crates/trusted-server-js/lib && npm run format
• Docs format: cd docs && npm run format
• WASM build: cargo build --package trusted-server-adapter-fastly --release --target wasm32-wasip1
• Manual testing via fastly compute serve: confirmed the inline flag, activation attribute, GPT page targeting, and decoded cust_params containing ts=true on a real GAM request
• Other: Cloudflare and Spin adapter tests/clippy, cross-adapter parity, native CLI tests, and Next.js/WordPress browser integration suites

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() added to production code
• Logging follows project conventions; no direct stdout/stderr logging was added
• New code has tests
• No secrets or credentials committed