Add HTTP request coalescing (in-flight deduplication)

[wille]

What

Adds request coalescing on the HTTP JSON-RPC path: identical concurrent read requests collapse into a single upstream call, and that one response is fanned back to every caller.

This reduces redundant load on upstream providers and lowers rate-limit (429) exposure under bursty traffic (e.g. many clients polling eth_getTransactionReceipt for the same hash).

How

• chain.Chain gains Coalesceable(method string) bool — a per-chain allowlist of read-only, idempotent methods. The EVM allowlist is shared by eth and tron; solana and btc have their own. Default-deny: anything not on the list is sent individually.
• core.Endpoint holds a singleflight.Group. A single coalesceable request runs through it, keyed by method + params (the JSON-RPC id is excluded since it varies per client).
• The provider failover loop is extracted into forwardWithFailover. The coalescing leader runs it under a detached context (context.WithoutCancel) so one client disconnecting can't cancel the shared upstream call the others are waiting on. Each caller writes a value-copy of the shared response stamped with its own id (never mutating the shared response).
• Batches and non-allowlisted methods bypass coalescing entirely.
• New haprovider_coalesced_requests_total{endpoint,method} metric; README updated.

Scope / non-goals

• HTTP only. WebSocket coalescing and batch coalescing are out of scope for now.
• Allowlist is deliberately conservative (default-deny): a read method not yet listed simply isn't coalesced until added.

Testing

• Unit + integration tests in internal/coalesce_test.go, driving the real handler concurrently against a hit-counting mock upstream:
  • 20 identical concurrent eth_call → 1 upstream hit; every client gets the result with its own id echoed.
  • 10 identical eth_sendRawTransaction → 10 hits (never coalesced).
  • Distinct params → not coalesced; batches → not coalesced.
• go test -race ./... green (stable across repeated runs); go vet and golangci-lint clean.