Preserve explicit denials when merging signed-media updates

When client state combines an initial signed-media map with later updates, distinguish an absent key from an explicit null denial. A truthiness fallback can silently keep an earlier granted link after the server withholds it. Preserve explicit-null entries across unrelated updates too; fixing selection precedence alone fails if the merge layer later discards the denial. Retain the authoritative server-render denial priority over older upload grants. Test both update orders and an unrelated event after denial. This corrects displayed availability; it does not revoke an already issued bearer URL before its expiry.

Applies when: authorized server responses and client events update shared temporary media references. Keywords: signed URLs React context null denial tombstone merge precedence expiry