Add a cluster maturity "Stage" panel to Charon Overview v3 #4503

Add a panel that classifies a cluster as Stage 0/1/2 so an operator can see at a glance whether the cluster is exposed to client-correlation risk, without manually counting peers in the Client Versions table.

Two stat panels next to "Cluster Info":

• Stage — big colour-coded Stage 0/1/2 tile (red/amber/green).
• Stage breakdown — three small tiles below it: <Client> BN centralisation, <Client> VC centralisation, Chain Split Halt. Each shows X/N and is coloured per the rules below.

Let T = cluster_threshold, N = cluster_operators, and X = max_observed_count + missing_peers (worst case — peers not reporting metrics are assumed to run the most-concentrated client).

• Stage 0 — X ≥ T for BN or VC (a single client family covers the threshold).
• Stage 1 — X < T for both BN and VC, AND every peer has the chain_split_halt feature flag enabled.
• Stage 2 — X ≤ N − T for both BN and VC, AND every peer has chain_split_halt.

If chain_split_halt isn't on every peer, Stage 1/2 are unreachable regardless of distribution.

• BN/VC tiles: red when X ≥ T, green when X ≤ N − T, amber in between.
• chain split halt tile: green only when enabled on every peer, otherwise red.