#1625 chore: Simplify shared Roslyn worker internals

Summary

• Separate shared Roslyn worker wire framing and template rendering from process lifecycle orchestration.
• Keep dynamic-code compilation behavior and worker protocol bytes unchanged.

User Impact

• Execute-dynamic-code compilation, retry, timeout, cancellation, and fallback behavior remain unchanged.
• The smaller worker host makes later lifecycle and assembly-build refactors easier to review independently.

Changes

• Add SharedRoslynCompilerWorkerProtocol to own request encoding, response header parsing, diagnostic framing, protocol read timeout, quit marker, and worker program template token replacement.
• Retarget existing command/template tests directly to the new owner and remove six host test wrappers.
• Add characterization coverage for valid response headers, invalid prefixes, and non-numeric exit codes before extraction.
• Reduce SharedRoslynCompilerWorkerHost from 988 to 865 lines while leaving retry, process lifecycle, assembly build, and shutdown state in the host.
• The only non-move production change is passing the current worker StandardOutput reader into ReadDiagnosticLines; all compile calls run under SharedCompilerWorkerLock, and Process.StandardOutput returns the same reader instance used for the response header.
• Preserve the existing double Path.GetFullPath application, blocking read behavior, strings, markers, timeout values, and out int exitCode contract.

Verification

• dist/darwin-arm64/uloop compile --project-path "$(git rev-parse --show-toplevel)" (0 errors, 0 warnings)
• SharedRoslynCompilerWorkerHostTests (10 passed)
• ExternalCompilerPathResolverTests (15 passed, including worker-build fallback)
• StaticFacadeStateGuardTests (20 passed)
• Confirmed that removed host wrappers have zero references and the protocol has no reverse dependency on the host.
• No IPC wire shape, CLI/package protocol version, or release input changes.