fix: expose stream write-contention error codes

[smiggleworth]

Summary

• Expose named FitzErrorCodes constants for stream stale-position conflict (2001) and active append-session admission contention (2002).
• Document their distinct meanings without changing wire decoding or retry behavior.
• Advance the package family to 1.4.2 while keeping assembly identity at 1.0.0.0.

Why

Portia must classify BeginAsync admission contention at its event-store boundary. The constants remove application-level magic numbers while preserving the broker's distinct semantics. This advances cntryl/portia#65.

Verification

• Focused wire-envelope tests: 14 passed.
• Release build with zero warnings and formatting verification passed.
• Fast test suites passed: core, analyzers, testing, and extensions.
• Packed 1.4.2 consumer restored from a fresh cache and ran successfully.
• Independent review confirmed the public API, protocol decoding, documentation, and packaging scope.

Fixes #38