Six recent push-to-main runs failed in `test_and_lint / Go test`. Each
failure hit a different test, but the same tests recurred — these are
races, not regressions. Fixes:

- stores/blockchain/sql: TestGenesisHashWrongParams mutated
  store.chainParams while the New() background goroutine read it.
  insertGenesisTransaction now takes *chaincfg.Params so the test passes
  wrong params without mutating shared state.

- services/blockassembly: TestBlockAssembly_GetMiningCandidate signalled
  completion before the assembler had wired the subtree into the mining
  candidate. Replace the WaitGroup with require.Eventually polling
  NumTxs == 3.

- services/blockassembly: TestHandleReorgWithInvalidBlock_Integration
  observed bestBlock=B3 during the window between BlockAssembler.reset()
  storing bestBlock and subtreeProcessor.Reset writing processed_at. The
  Eventually now also requires processed_at to be flushed on every chain
  B block before downstream assertions run.

- services/blockvalidation: Test_consumerMessageHandler/context_cancellation
  raced ctx.Done() vs a buffered channel send: when blockHandler returned
  nil quickly, the buffered send succeeded and select picked errCh=nil.
  Cancel before constructing the handler and use an unbuffered channel so
  the handler's select can only fire ctx.Done().

- util/kafka: TestPerf* spin up a Redpanda testcontainer and measure
  throughput. They are not unit tests, blow the per-job time budget under
  -race, and the wait condition is a log line that fires before the admin
  API is reliable. Gate behind //go:build perf so make test skips them;
  run them with go test -tags perf.

- CI: disable testcontainers' Ryuk reaper sidecar in PR/main test
  workflows. Pulling testcontainers/ryuk:* from Docker Hub on shared
  runners is the dominant flake source for tests using the aerospike
  testcontainer (Test_processTransactionInternalAerospike, the validator
  TestExtendedTx*). Tests already call container.Terminate() explicitly,
  so the reaper provides no additional safety on ephemeral runners. Also
  set TESTCONTAINERS_RYUK_DISABLED in test/utils/aerospike/aerospike.go
  for local dev convenience.

Out of scope: TestDaemon_Start_AllServices needs a real getFreePort
TOCTOU fix — left for a follow-up.