Adds deterministic regression coverage for the network-layer race fixes from #8633. Each new test was verified to fail (assertion failure or -race report) with the corresponding fix reverted, and to pass with it.

Changes

• network/slashing: first tests for the package. Verify the violations consumer does not mutate the caller's Violation and that one violation can be reported concurrently through all entry points.
• RPC inspector: verify async inspection does not mutate the inspected RPC (the publish message list stays byte-for-byte in place), since libp2p pubsub keeps using the RPC concurrently.
• ALSP SpamRecordCache: same-record concurrent Get/AdjustWithInit test. The existing concurrency tests only exercised disjoint records and did not catch the race. Also verifies the returned penalty is the one produced by the caller's own adjustment.
• AppSpecificScoreCache: same-peer concurrent update/read test, for the same reason.
• Scoring registry: unit test for the silence-period state machine, including the new "registry not started yet" branch (previously the zero start time spuriously ended the silence period before startup).

Related: #8633

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