framework: guard executor shared state under stateMu

[nadahalli]

The race

The background refresh loop (EnsureFreshEnclaves) and getLocalNodeAndCapConfig
rewrite shared executor state while Execute reads it concurrently. Only
donMembers/donF were guarded, by donMu.

Left unprotected:

• nodeID (string): written in getLocalNodeAndCapConfig, read in Execute's
  scoped emitter. A torn string read pairs a new pointer with an old length.
  Memory-unsafe, not just stale.
• enclaves ([]types.Enclave): written by the refresh loop, read by GetEnclaves.
• vaultDON.CryptographyThreshold: written by the refresh loop, read in
  GetEncryptedDecryptionShares.

enclaveClient is NOT part of this. It is published once in initLazily under
initializedMutex.

The fix

Renames donMu to stateMu, moves nodeID and enclaves under it, and routes access
through getters/setters. vaultDON.CryptographyThreshold is guarded via
getVaultThreshold/setVaultThreshold; the rest of vaultDON is publish-once at init.

MockMetrics.Emit is guarded so the harness is not itself the race.

Verification

TestExecutor_ConcurrentRefreshAndExecute_NoDataRace: 8 goroutines x 40 iterations
across Execute / EnsureFreshEnclaves / GetEnclaves.

Reverting executor.go to main while keeping the test reproduces it:

WARNING: DATA RACE
Write at 0x00c0001c6fb8 by goroutine 42:
  (*RealExecutor).getLocalNodeAndCapConfig()
  (*RealExecutor).EnsureFreshEnclaves()
Previous read at 0x00c0001c6fb8 by goroutine 53:
  (*RealExecutor).Execute()

Full framework suite green under -race.