Improve error messages: unimported module qualifiers and nominal-vs-record mismatches

[jaredramirez]

Two error-reporting improvements, one commit each.

1. Clearer error when a qualified ident's module or type cannot be found

Previously, using Stdin.line!() without import pf.Stdin reported only:

┌────────────────┐
│ DOES NOT EXIST ├─ Stdin.line! does not exist.

which reads as "this function doesn't exist" when the real problem may be a missing import. When a qualified ident fails to resolve because its leading qualifier is neither an imported module nor a type in scope, canonicalization now keeps the existing qualified_ident_does_not_exist diagnostic and DOES NOT EXIST title while adding clearer context to the body:

┌────────────────┐
│ DOES NOT EXIST ├─ `Stdin.line!` does not exist.

    The name `Stdin` is not an imported module or a type in scope.

    If `Stdin` is a module, you may need to import it at the top of the file.
    For example:

        import pf.Stdin

The suggested import uses the file header's platform shorthand (or first package shorthand) when one exists, and falls back to a bare import Stdin otherwise. The diagnostic carries explicit lookup context: the missing-module-or-type path records the unresolved name and complete suggested import, while the existing missing-exposed-value path has its own context and retains the shorter report body. This avoids a duplicate unresolved-qualified-identifier diagnostic tag while keeping the renderer's behavior explicit.

2. Field-level hints for nominal-vs-record type mismatches

A record that mismatches a nominal annotation like

ToolDefinition(input) := { name : Str, description : Str, call : input -> Str }

previously showed only the two types with no hint. The snapshot type diff now opens a nominal type on exactly one side of a structure-vs-structure comparison and compares its declaration backing, so the usual record hints apply:

    Hint: This record is missing the field: `call`

This is gated on the backing being visible from the module reporting the error — the nominal type is transparent, or it was declared in that module — mirroring types.NominalType.canLiftInner (the same rule as whether a nominal type can be opened). SnapshotNominalType now carries is_opaque to make that check possible at report time.

Testing

• New snapshot tests: test/snapshots/can_import_missing_import_suggestion.md, test/snapshots/nominal/nominal_record_mismatch_field_hint.md; existing snapshots regenerated.
• zig build run-test-zig-module-can passes, and all affected snapshots pass --check-expected --check-output validation.
• The earlier full run-test-zig run passed (3432 passed / 0 failures). The json_decoder / http_header_decoder platform steps fail with an ARC-certifier panic, which I verified is byte-identical on a clean checkout of main (pre-existing, unrelated).

🤖 Generated with Claude Code

https://claude.ai/code/session_01DGo1pdehucvHBTSoGBm22n