fix: type assertion property access warning to error + loc propagation

[cs01]

Summary

• Property access on type assertion expressions that can't be resolved now produces a compile error instead of silently returning 0.0 (wrong value at runtime)
• Synthetic member access expressions created during type assertion resolution now carry source location info for accurate error messages
• Fixed redundant as VariableDeclaration casts in escape-analysis.ts that were silently returning 0.0 for .loc access

Changes

• member.ts: Added loc: expr.loc to both synthetic expression creation sites in handleTypeAssertionPropertyAccess
• member.ts: Converted emitWarning + return "0.0" to emitError for unresolved type_assertion properties
• escape-analysis.ts: Removed redundant type assertions that triggered the type_assertion property handler

Test plan

• All tests pass
• Self-hosting Stage 0 + Stage 1 pass