docs: change-doc for Alchemy bundler rollout (cuts v4.4.0 pre-release)

docs/changes/20260720-alchemy-bundler-provider.md

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+# Alchemy bundler routing via `bundler_provider`
+
+**Date:** 2026-07-20
+
+## Summary
+
+Adds a per-chain `bundler_provider` switch (`alchemy` | `self_hosted`, default
+`alchemy`) so UserOps can be relayed through Alchemy's Rundler instead of the
+self-hosted Voltaire bundler, with no change to the paymaster (still our
+per-chain `VerifyingPaymaster`, per-chain deposits — only the relay moves).
+
+## What changed
+
+- `SmartWalletConfig` gains `BundlerProvider` + `AlchemyAPIKey`;
+  `ActiveBundlerURL()` resolves the endpoint (alchemy URL derived from
+  `AlchemyAPIKey` + a chainId→subdomain map; `self_hosted` uses `bundler_url`).
+  The alchemy path never falls back to `bundler_url` — a missing key or unmapped
+  chain is a hard error (fail closed). `BundlerConfigured()` replaces the old
+  `bundler_url != ""` connectivity-only guard.
+- Two ERC-4337 portability fixes for Rundler: `eth_simulateUserOperation` (a
+  Voltaire-only method) is now best-effort; and verification gas is taken from
+  the real estimate rather than the 1M default, which tripped Rundler's
+  verification-gas-limit-efficiency floor.
+- Subdomain map covers Ethereum (1), Base (8453), Sepolia (11155111),
+  Base-Sepolia (84532), BNB (56 → `bnb-mainnet`).
+
+## Rollout
+
+Config ships via `avs-infra` (`sync-configs.sh --apply`) separately from the
+code release. Every wallet-op chain sets `bundler_provider` explicitly and
+`ALCHEMY_API_KEY` is a Railway sealed var. BNB stays `self_hosted` until it has a
+real deployed + funded paymaster on BNB mainnet (its config paymaster is a
+placeholder today, so making `BundlerConfigured()` true would fail the fatal
+startup paymaster probe).
+
+Validated end-to-end on Sepolia: a real UserOp lands through both providers.