TEST-007: Add test samples for struct field access behavior

[Raulgooo]

Summary

src/typecheck.c:165 has a comment: // Bind fields as unknown for now (full struct field lookup is TODO). No tests document this behavior.

Scope

Add two .runes files to src/tests/samples/:

1. Positive test: uses struct.field syntax — should compile (field type is currently TY_UNKNOWN, that's expected for now)
2. Negative test: accesses a nonexistent field name — should fail with -- EXPECT FAIL: field

Acceptance Criteria

• Positive test added and passes tester.bash
• Negative test added with -- EXPECT FAIL: field on line 1 and passes as XPASS
• Both files document the TODO behavior in a comment