[type-system] deduplicate fieldTypeToLlvm and add fail-loud default

[cs01]

Before

fieldTypeToLlvm was duplicated identically in class.ts and control-flow.ts, each with a silent return "i8*" fallback that hides unrecognized types.

After

No user-facing change. Internal refactor only.

Single fieldTypeToLlvm in type-system.ts with explicit handling for every known type category (primitives, arrays, collections, unions, opaque types, function types, class/interface names) and throw on truly unrecognized types.

Description

• Extracted shared fieldTypeToLlvm() from type-system.ts (29 LOC)
• class.ts and control-flow.ts now delegate to shared function via thin wrappers that handle enum check locally (enum check uses this.isEnumType() which requires class context)
• Net -8 LOC (41 insertions, 45 deletions across 4 files)
• Also deduplicates methodReturnTypeToLlvm union handling would be a behavioral change, so left as-is for a separate coordinated PR

Partial progress on #710.