statement-store: first v2 DHT e2e test and opt-in CLI flags

[DenzelPenzel]

Implementation (#12428)

What

Lands the first end-to-end test for the v2 DHT-affinity statement routing path,
plus the minimal wiring required to exercise it from zombienet.

Changes

• Make the v2 path opt-in. v2dht_enabled() was a hard-coded const fn.
  It is now a per-node runtime flag (StatementHandler::v2dht_enabled), threaded
  from a new CLI option --enable-statement-store-v2-dht (default off).
• Expose topology parameters as CLI flags:
  • --statement-replication-factor <K>
  • --statement-gossip-target <N>
• New zombienet test v2_dht_k1_subscribe_submit_resubscribe
  on a 2-node network with K=1 it asserts
  1. the subscriber receives the submitted statement (delivery),
  2. exactly one node stores it locally (count_storers == 1, selectivity),
  3. after dropping and recreating the subscription, a second statement is
     delivered again (resubscribe stability).
• Test helpers in common.rs: collator_args_v2, stores_locally,
  count_storers.

[DenzelPenzel]

/cmd fmt

[paritytech-workflow-stopper]

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