refactor: Workerize message signing

[mondoreale]

Summary

Offload message signing to a Web Worker to avoid blocking the main thread during cryptographic operations. This mirrors the existing signature validation workerization pattern.

Changes

• Extract SigningService from MessageSigner for clearer separation of concerns
• Add SigningWorker and signingUtils for worker-based signing
• Add platform-specific worker factories (_nodejs, _browser, _jest, _karma)
• Extend Identity with getPrivateKey() method to detect workerizable identities
• Update MessageSigner to use worker when identity provides a private key, falling back to direct signing for ERC-1271
• Consolidate worker bundling in rollup config with declarative WORKERS registry
• Inline browser worker dependencies to avoid chunk copying issues
• Export DestroySignal and SigningService from SDK for external usage
• Add createMockSigningService() and createMessageSigner() test utilities
• Register SigningService in FakeEnvironment for test compatibility

Limitations and future improvements

• External wallet identities (EthereumProviderIdentity) cannot be workerized as they require browser UI interaction
• Worker startup has minor overhead; could consider worker pooling for high-frequency signing scenarios

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Note

Medium Risk
Touches core message signing flow and adds worker lifecycle/build configuration; regressions would affect publishing/signature generation across environments.

Overview
Moves message signing onto a dedicated worker thread via new SigningService/SigningWorker, with MessageSigner choosing the worker path when the Identity exposes a private key (and falling back to direct signing for ERC_1271).

Adds platform-specific createSigningWorker factories and updates build/test tooling (Rollup worker registry + inlined browser worker bundles, Jest/Karma module mapping, and increased timeouts). Tests are refactored to use new helpers (createMessageSigner/mock signing service), FakeEnvironment now registers a shared SigningService, and the SDK exports SigningService and DestroySignal.

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