feat(factory, farming-pool): report refreshed pool IDs, add get_pool_info, verify credit-rate event (#393, #394, #395, #396)

[aurorabini]

Closes #393
Closes #394
Closes #395
Closes #396

Summary

One commit per issue. Two factory TTL changes and two farming-pool
readability/coverage changes, with no behaviour change beyond the two
additive return values described below.

#393 — refresh_pool_ttls reports what it refreshed (ddf4646)

The existence check was already in place, so IDs without a record were
skipped — but the entry point returned Ok(()), leaving callers no way to
tell which IDs were actually kept alive versus skipped. A keeper
reconciling a sweep against a known registry had to guess.

• New RefreshPoolTtlsResponse: refreshed (ascending IDs that existed and
  were bumped), end_id (clamped resume point) and missing (count skipped
  for an absent record).
• refresh_pool_ttls now returns it. The existence check and the ttl_ref
  event payload are unchanged, so the event contract stays stable.
• The two existing refresh_pool_ttls tests were asserting Ok(Ok(())) and
  are updated to the new return value.

#394 — coverage for missing pool records (52413c9)

• test_refresh_pool_ttls_reports_only_existing_pools_and_counts_gaps: pool
  IDs are handed out sequentially from PoolCount, so a hole inside the
  sweep range means a record was removed from storage (TTL lapse /
  archival) — the same case list_pools reports as pool_gap. The test
  archives the middle record, ages the survivors, and asserts the sweep
  returns only the survivors, reports missing == 1, extends their TTLs to
  TTL_EXTEND_TO, and does not materialise the archived record.
• test_refresh_pool_ttls_reports_empty_sweep_past_the_registry: a
  start_id beyond the count sweeps nothing and reports an empty result with
  end_id clamped, instead of an ambiguous unit return.

#395 — get_pool_info aggregate query (bbcb745)

A pool overview needed up to six separate calls (total_staked,
credit_rate, global multiplier, min lock period, min stake amount, paused
flag), each paying its own invocation cost and TTL bump.

• New PoolInfo contract type and get_pool_info(), reading instance
  storage directly behind require_initialized + bump_instance like every
  other getter.
• Tests assert the aggregate agrees with every individual getter it
  replaces, and that an uninitialized pool returns NotInitialized rather
  than a zeroed PoolInfo that would be indistinguishable from an empty pool.

One deliberate deviation: the issue also lists "number of stakers". There
is no staker-count entry in storage — it is derived by paging get_positions
— so a field here would report a value nothing maintains. The two maintained
credit counters (total_banked_credits, total_distributed_credits) are
included instead, so the overview is still a single round trip. This is
noted in the code comment and the commit message.

#396 — verify the set_credit_rate event (dfd04ba)

The issue asks to verify the event already includes both rates. It does:
set_credit_rate publishes (old_rate, new_rate, ledger_sequence). This
adds the missing verification, and it verifies the property that actually
matters — across a sequence, not a single change. Three consecutive
changes (10 → 25 → 40 → 5) are pinned as three rate_set events, where each
event's old_rate equals the previous event's new_rate. That chain is what
lets an indexer reconstruct rate history and compute deltas without reading
storage. The final value is also asserted against the credit_rate getter.

Notes for maintainers

• Both contract changes are additive: a new response type on an existing
  entry point and a new read-only entry point. No storage keys, no
  authorization changes, and no existing call sites outside the tests.
• min_lock_period is returned in ledgers, matching the internal
  read_min_lock_period and the existing min_lock_period() getter, rather
  than the seconds variant.
• Verified by reading in this environment (no cargo builds or test runs
  performed here, per the contribution constraints) — CI should compile and
  execute the suites.

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