compiler: improve diagnostics and multiline literal union parsing

[eggplantiny]

Context

Derived from the 2026-04-25 and 2026-04-28 Claude Code usage review.

Claude Code reported compiler errors such as internal parser/type mismatch codes without enough user-oriented explanation, and multiline literal union syntax appeared to be rejected even though literal-only unions are semantically supported.

Current assessment

SPEC status: diagnostic structure is provided, but quality may be insufficient. Literal-only unions are supported semantically; multiline parsing appears to be parser/DX work rather than a new semantic feature.

CompileDiagnostic already has structure for severity, code, message, location, source, and optional suggestion. The implementation should fill these fields with intent-oriented messages and suggestions where possible.

Desired outcome

Improve compiler diagnostic messages/suggestions and accept line breaks in literal-only union syntax where the grammar should allow it.

Acceptance criteria

• Diagnostics for common parser/type errors include actionable messages and suggestion when useful.
• Multiline literal-only union syntax is parsed or explicitly documented if intentionally unsupported.
• Tests cover type X = "a" | "b" | "c" across multiple lines.
• Non-trivial arbitrary unions remain rejected unless a separate SPEC change authorizes them.