Complete removal of the legacy consent KV path

[ChristianPavilonis]

Summary

• Remove the inactive legacy consent KV fallback/write path, storage module, source metadata, and Fastly route dependency.
• Keep live consent request-local and preserve ec.ec_store as the sole KV-backed EC identity/withdrawal store.
• Fail fast when stale TOML or JSON/app-config still contains consent_store, with explicit operator migration guidance.

This PR is stacked on #902.

Changes

Area	Change
Consent core	Remove KV fields/branches from ConsentPipelineInput, delete the dead storage module, and remove ConsentSource::KvStore
Configuration	Remove ConsentConfig::consent_store and add strict TOML/runtime-JSON rejection tests
Fastly	Stop opening an unused consent store for auction, page-bids, and publisher routes; remove the local fixture and named opener
Other adapters	Correct generic platform-KV comments that still advertised consent persistence
Documentation	Explain request-local consent authority, strict upgrade failure, binding cleanup, rollback retention, and why legacy records are not migrated into ec.ec_store

Migration note

Deployments that still configure [consent].consent_store must remove the field before upgrading. Strict config loading intentionally rejects it. The old store may be retained unchanged for a rollback window, then unlinked and deleted; its records must not be copied into ec.ec_store.

Closes

Closes #883

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
• 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 — not run
• Other: strict TOML/JSON migration tests, consent and Fastly dispatch tests, cargo test-cloudflare, cargo test-spin, parity, ./scripts/test-cli.sh, all Cloudflare/Spin clippy targets, and all-features core rustdoc

Checklist

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