cl: archive nodes advertise CUSTODY_REQUIREMENT while custodying all column groups

An archive node custodies every column group but advertises `CUSTODY_REQUIREMENT`.

`IsArchivedMode` is true when `--caplin.blobs-archive` or `--caplin.blobs-immediate-backfill` is set (`cl/das/peer_das.go:262`), and `resubscribeGossip` then subscribes to all `DataColumnSidecarSubnetCount` (128) subnets (`:295-299`). Confirmed on a gnosis archive node: 128 `JOIN /eth2//data_column_sidecar_N` topics, and blob reconstruction from gossiped columns succeeds at the tip — 37 `[blobsRecover] recovering blobs` with 37 matching `recovering done`, no `not enough columns to recover`.

The ENR and the metadata response tell a different story. The only non-mock path that raises the count is `UpdateValidatorsCustody` → `SetCustodyGroupCount`, whose sole caller is `on_block.go:440` inside the validator-custody branch. Without validators attached it never runs, so `advertisedCgc` stays at `CustodyRequirement` for the process lifetime — 4 on every network, since no chain overrides it. `metadataV3Handler` reports the same value (`cl/sentinel/handlers/heartbeats.go:116`), so both discovery and the metadata RPC understate custody by 32x.

The cost is borne by the network rather than the node: peers pick column sources from the advertised count, so a node holding all 128 groups is asked for 4 and its data goes unused. The node itself still receives everything, because its gossip subscriptions come from `GetMyCustodyColumns`, not from what it advertises — which is why this is invisible in normal operation.

The fix is presumably for archived mode to advertise `NumberOfColumns` rather than leaving the seeded requirement in place, but the interaction with the increase-only ratchet and with #23566 wants deciding together rather than piecemeal.

Found in review of #23560, which seeds the entry (0 -> 4 on the node above) and deliberately stops short of this.

Start with IsArchivedMode and resubscribeGossip in cl/das/peer_das.go, then trace SetCustodyGroupCount in on_block.go and metadataV3Handler in cl/sentinel/handlers/heartbeats.go. Review #23566 and #23560 before deciding how archived-mode advertising interacts with the increase-only ratchet. Done means archive nodes advertise their actual custody consistently in the ENR and metadata response, with coverage for the chosen behavior.

Written by the indexing model from the issue text.