Handle platform alias error row extensions

[lukewilliamboswell]

Fixes #9655.

Summary

• Treat artifact-blocking invalid type-declaration diagnostics as runtime errors so coordinator relation validation is skipped on user errors.
• Demonstrate supported platform-exposed callback/error aliases with explicit named tag-union extension parameters.
• Add fx-open coverage showing the platform catches non-Exit errors with Str.inspect and exits with a generic non-zero code.
• Improve the anonymous .. type-declaration diagnostic to point users at the named extension-parameter form.

Testing

• zig build run-test-zig-module-compile -- --test-filter "issue 9655"
• zig build run-test-zig-module-can -- --test-filter "open ext"
• zig build run-test-cli -- --suite subcommands --filter "fx-open platform alias"
• zig build run-test-cli -- --suite platforms --filter "fx-open"
• zig build run-check-test-wiring
• zig build run-check-fx-platform-test-coverage --summary all --color off
• zig build run-check-zig-format
• zig build run-check-snapshots
• git diff --check
• ./zig-out/bin/roc check test/fx-open/open_error_alias.roc
• ./zig-out/bin/roc --opt=interpreter --no-cache test/fx-open/open_error_alias.roc

Note: local zig build minici is blocked by unrelated run-test-dylib/run-test-archive DCE canary failures (ROC_DCE_CANARY_BLOB_7f3a9c), which also fail when rerun directly.