feat!: migrate RPC methods to context-first signatures

[neverDefined]

Phase 2 PR C — the largest breaking change in the roadmap. Every mutating RPC wrapper now takes `context.Context` as its first parameter, so callers can enforce per-call deadlines and cancellation.

Summary

Breaking (pre-1.0; will bump the next minor tag):

• 18 methods gain a leading `ctx context.Context`: `MineBlock`, `SetNextBlockTimestamp`, `IncreaseTime`, `SetBalance`, `Impersonate`, `StopImpersonating`, `Snapshot`, `Revert`, `SetCode`, `SetStorageAt`, `SetNonce`, `Mine`, `DropTransaction`, `SetAutomine`, `SetIntervalMining`, `AutoImpersonate`, `ResetFork`, `ResetState`.
• Internal calls migrated from `rpcClient.Call(nil, ...)` to `rpcClient.CallContext(ctx, ...)`.
• `EthereumTestEnvironment` interface updated to match.

Additive:

• `(*Anvil).WaitForMemPoolEmpty(ctx, timeout)` — method variant that honors both ctx and an explicit timeout; uses the instance's own client.

Deprecated:

• Free function `MemPoolEmpty(ctx, client)` — kept as a shim for one release with `// Deprecated:` godoc tag.

Docs updated:

• `CHANGELOG.md` — Changed / Added / Deprecated sections with a migration snippet.
• `CLAUDE.md` — replaced the "pending migration" note with the current API shape and a template for new RPC wrappers.
• `README.md` — Quick Start and every Usage Examples snippet updated to pass `ctx`.

Migration

```go
// before
anvil.MineBlock()
anvil.SetBalance(addr, bal)
snapshotID, _ := anvil.Snapshot()

// after
ctx := context.Background() // or t.Context() in tests, or ctx with a timeout
anvil.MineBlock(ctx)
anvil.SetBalance(ctx, addr, bal)
snapshotID, _ := anvil.Snapshot(ctx)
```

Test plan

• `go build ./...` clean
• `go vet ./...` clean
• `golangci-lint run ./...` — 0 issues
• `go test -race ./...` — green locally, 30s, against anvil 1.5.0 (includes 2 new WaitForMemPoolEmpty subtests: success path + ctx-cancel path)
• CI: all three jobs (test / vuln / lint) green on the matrix

Scope notes

• Pre-existing examples outside this repo will need to add `ctx` arguments — the one migration snippet above covers the transformation.
• `WaitForBlock` already takes a `timeout` and constructs its own ctx internally; its signature is unchanged.
• Pure Go getters (`Client`, `RPCClient`, `Metrics`, `Accounts`) do not take ctx — they don't call RPC.
• Lifecycle methods (`Start`, `Stop`, `Close`) are unchanged.

Closes #36
Closes #39

Follow-ups in this phase

• PR D (Phase 2 polish): `[phase-2] replace 2s startup sleep with readiness probe #37` readiness probe, `[phase-2] route anvil subprocess stdout/stderr through zerolog #38` stdout routing, `[phase-2] verify executability of fallback anvil binary path #40` anvil executable check, `[phase-2] decide: use or remove the EthereumTestEnvironment interface #41` interface decision.
• PR E: `[phase-2] CI matrix (os + go) and upload coverage to codecov #42` CI matrix + codecov, `[phase-2] add examples/ folder and godoc Example functions #43` examples/ folder (examples can now be written against the final API).

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