fix(ios): preserve specific protocol errors during dispatch

Summary

• Preserve every declared BridgeError through handler narrowing using the error set itself, then translate implementation aliases and retain the unknown-error fallback.
• Table-test the full error vocabulary at the mapper and iOS dispatcher boundary.
• Regress both missing-WebView presenter lookups and carry a real share-handler failure through narrowing and correlated JSON serialization.

Closes #271.

Validation

• The table-driven test failed before the fix because WindowHandleNotSet became NativeCallFailed; WebView and other explicitly declared variants had the same defect.
• All 34 bridge-error/request-context unit tests pass after the fix.
• The complete native suite passes in an isolated checkout using the locked Zig toolchain and CI-pinned first-party dependencies.
• Pinned Zig format checks and git diff checks pass.