9467789fc16e972b2496b0f4e854bbd925b14c55  Advertise stored compression dictionaries and decode dcb and dcz responses

Classified as not security-relevant.
Triage note: Implements the fetch compression-dictionary algorithm: shouldFetchWithCompressionDictionary (mode/cookie/secure-context gating), best-match retrieval in NetworkCache, removeHTTPHeaderField, opaque-tainting blocking, and libsoup wiring. The security checks (opaque-response block, same-origin) are part of the new feature's spec compliance, not a fix of a pre-existing vulnerability.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), null_guard (+2.59), uaf_lifetime (+4.75), large_churn (-2.0)