[sema] reject inline as { ... } on opaque source types

[cs01]

Before

(x as unknown as { type: string; value: number }) silently passed the type-assertion prefix checker because there's no source interface to compare against. In native code, the inline fields become GEP indices — if they don't match the actual struct layout, it reads wrong memory.

After

Compile error when an inline as { ... } assertion (2+ fields) follows an as unknown, as any, or as object intermediate cast. Suggests using as NamedInterface instead.

Description

Extends type-assertion-checker.ts with a new checkOpaqueSource method. This was the last unaccounted-for hazard in the "Patterns That Crash" list — inline as { ... } on opaque-typed sources where the prefix checker has nothing to verify against. ~30 LOC.

Closes item #3 from #729. Replaces #747 which was closed without merge due to stale CI check status.

Next Steps

• Item installer updates #1 from [sema] auto-enforce native compiler pitfall rules #729 (alloca for collection structs in class fields) confirmed safe — no sema pass needed
• Item add subcommand support to argparse and migrate chad-native.ts #6 (interface method dispatch) confirmed working — no sema pass needed
• Remaining: items add --target-cpu flag and use x86-64 in ci to fix sigill on runners #2, implement try/catch/finally via setjmp/longjmp #7, Add file embedding and multi-column SQLite support #8 (all lower priority)