Support Sourcepoint GPP consent for EC generation

[ChristianPavilonis]

Summary

• Add client-side Sourcepoint JS integration that auto-discovers _sp_user_consent_* from localStorage and mirrors GPP consent into __gpp / __gpp_sid cookies
• Extend server-side GPP decoding to extract sale_opt_out from US GPP sections (IDs 7–23)
• Add GPP US consent branch to allows_ec_creation() between existing TCF and us_privacy checks

Closes #640

Test plan

• Rust tests pass (cargo test --workspace — 992 tests including 8 new)
• JS tests pass (npx vitest run — 288 tests including 6 new)
• Clippy clean
• Verify EC generation succeeds for a Sourcepoint-only site in a regulated US state with GPP consent present
• Verify GPC still blocks EC when GPP US section is permissive
• Verify existing TCF and us_privacy EC gating behavior unchanged

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