Centralize DOM insertion guards

[prk-Jr]

Summary

• Centralize SDK DOM insertion interception behind a single shared dispatcher so integrations no longer stack independent appendChild / insertBefore wrappers.
• Move GPT onto the shared dispatcher and harden its proxy path so dynamic GPT loads remain first-party without broadening upstream encoding negotiation.
• Add coordination and reset coverage so SPA re-inits, test runs, and future integrations do not accumulate prototype patches.

Changes

File	Change
crates/common/src/integrations/gpt.rs	Switched GPT asset fetching to the shared proxy helper, preserved caller Accept-Encoding, updated bootstrap comments, and refreshed proxy tests/fixtures.
crates/js/lib/src/shared/dom_insertion_dispatcher.ts	Added the runtime-global DOM insertion dispatcher that owns the single appendChild / insertBefore patch, orders handlers deterministically, and restores baselines safely.
crates/js/lib/src/shared/script_guard.ts	Refactored shared script guards to register dispatcher handlers instead of patching DOM prototypes directly.
crates/js/lib/src/integrations/gpt/script_guard.ts	Moved GPT Layer 5 onto the shared dispatcher and coordinated reset through handler unregistering.
crates/js/lib/src/integrations/gpt/index.ts	Made GPT shim activation robust when the inline enable flag executes before the unified bundle.
crates/js/lib/src/integrations/datadome/script_guard.ts	Updated DataDome to the new shared guard API and aligned guard comments with dispatcher behavior.
crates/js/lib/src/integrations/google_tag_manager/script_guard.ts	Updated GTM to the new shared guard API with stable integration identity.
crates/js/lib/src/integrations/lockr/nextjs_guard.ts	Updated Lockr to the new shared guard API and aligned guard comments with dispatcher behavior.
crates/js/lib/src/integrations/permutive/script_guard.ts	Updated Permutive to the new shared guard API with stable integration identity.
crates/js/lib/test/shared/dom_insertion_dispatcher.test.ts	Added shared-dispatcher coverage for single patch ownership, ordering, reset coordination, external patch ownership, handler isolation, and repeated install/reset cycles.
crates/js/lib/test/integrations/gpt/index.test.ts	Added coverage for GPT auto-install when the enable flag is set before module evaluation.
crates/js/lib/test/integrations/gpt/script_guard.test.ts	Updated GPT guard tests for dispatcher-based reset behavior and realistic versioned GPT asset paths.
crates/js/lib/test/integrations/datadome/script_guard.test.ts	Updated DataDome guard tests to assert reset restores shared prototype baselines without manual prototype cleanup.
crates/js/lib/test/integrations/google_tag_manager/script_guard.test.ts	Updated GTM guard tests to assert reset restores shared prototype baselines without manual prototype cleanup.
crates/js/lib/test/integrations/lockr/nextjs_guard.test.ts	Updated Lockr guard tests to assert reset restores shared prototype baselines without manual prototype cleanup.

Closes

Closes #402

Test plan

• cargo test --workspace
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• JS tests: cd crates/js/lib && npx vitest run
• JS format: cd crates/js/lib && npm run format
• Docs format: cd docs && npm run format
• WASM build: cargo build --bin trusted-server-fastly --release --target wasm32-wasip1
• Manual testing via fastly compute serve
• Other: cd crates/js/lib && npm run build

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() in production code — use expect("should ...")
• Uses tracing macros (not println!)
• New code has tests
• No secrets or credentials committed

[aram356]

Summary

Well-executed refactoring that replaces N independent appendChild/insertBefore prototype patches with a single shared dispatcher. The JS design is sound and thoroughly tested. The Rust-side GPT proxy rewrite introduces two behavioral changes that need attention before merge.

Blocking

🔧 wrench

• Redirect following widens proxy trust boundary: build_proxy_config inherits follow_redirects: true, allowing the GPT proxy to follow up to 4 redirects to any host. The old code was single-hop. GPT static assets don't redirect, so follow_redirects = false is the correct setting. (gpt.rs:128)
• Error handling semantics changed silently: Old code returned Err on non-2xx upstream status; new code passes through the response body. This is arguably better behavior but is a semantic change that should be documented or reverted. (gpt.rs:160)

Non-blocking

🤔 thinking

• Vary header normalization dropped: Old vary_with_accept_encoding logic removed; relying on upstream Google CDN to always send correct Vary headers. Low practical risk but removes a defensive layer. (gpt.rs:147)
• Privacy-scrubbing relies on header set-last-wins: copy_proxy_forward_headers forwards Referer/X-Forwarded-For before with_header overrides blank them. Correct today, fragile if the proxy helper ever appends instead of sets. (gpt.rs:134)
• Stale dispatcher state replaced without teardown: Version mismatch path overwrites the global without restoring old prototype wrappers. No production risk (single page load), but can stack wrappers in HMR/dev. (dom_insertion_dispatcher.ts:81)

♻️ refactor

• handle JSDoc ambiguous about node insertion: "Consumed" implies the node might not be inserted, but the dispatcher always inserts. Clarify that return value only controls handler chain short-circuiting. (dom_insertion_dispatcher.ts:28)

👍 praise

• Single shared prototype patch: The core dispatcher design eliminates wrapper stacking, makes ordering deterministic, and the test coverage (priority ordering, external patch preservation, repeated cycles, error resilience) is thorough.
• GPT auto-install on pre-set flag: Closes a real race condition between inline bootstrap and bundle evaluation. Both orderings tested.
• Test cleanup delegated to dispatcher teardown: Tests validate that teardown works rather than masking broken cleanup with manual prototype restoration.

CI Status

• cargo fmt: PASS
• cargo test: PASS
• vitest: PASS
• format-typescript: PASS
• format-docs: PASS
• CodeQL: PASS

[ChristianPavilonis]

Key items to address:

• Keep proxy redirect handling constrained in crates/common/src/integrations/gpt.rs (do not broadly follow redirects without explicit allow-listing/trust guarantees).
• Clarify and stabilize GPT proxy error semantics in crates/common/src/integrations/gpt.rs (the non-2xx passthrough change needs either rollback or explicit documentation/tests).
• Restore explicit cache/header handling in crates/common/src/integrations/gpt.rs (Vary normalization and deterministic privacy header scrubbing should not rely on set order).
• In crates/js/lib/src/shared/dom_insertion_dispatcher.ts, ensure stale dispatcher replacement does best-effort teardown in version mismatch paths and tighten handle() JSDoc return semantics.

Once these are addressed (or explicitly accepted with rationale), I am happy to re-review.