fix(codegen): fail closed on discarded lowering errors

[slepp]

Summary

Silent .ok()? swallows in the lowering/codegen pipeline are removed. A classification error in the owned-aggregate value-class path now propagates: the type is excluded from the drop allow-set and a diagnostic is emitted, rather than silently resolving to "not an owned aggregate" and skipping drop synthesis. A spawn-state lowering error now guarantees a diagnostic instead of returning a bare None. The DWARF base-type and field-index sites—infallible by construction—use .expect() with an invariant note. One symbol-classifier site retains .ok()? with a // JUSTIFIED: comment: a malformed suffix is a benign non-match, not a swallowed error.

Verification

• make test-hew-ratchet: pass — 0 expected failures / 0 actual failures; ratchet PASSED (load-bearing check for the drop/value-class surface)
• make build: pass — all crates compiled
• cargo fmt -p hew-mir -p hew-codegen-rs -- --check: pass
• cargo clippy -p hew-mir -p hew-codegen-rs --all-targets: pass — zero warnings
• ast-grep scan --rule rules/rust/fail-closed/ok-question-in-lowering.yml: 0 unannotated hits
• Full ast-grep lint gate (scripts/ast-grep-lint.sh): pass — 0 error-severity matches
• Preflight: ready-to-push

Out of scope

• The two new defense-in-depth diagnostic branches (classification-error reason in the owned-aggregate value-class path; spawn-state NotYetImplemented fallback) do not yet have dedicated reject fixtures. These branches guard internal-invariant conditions that are not currently reachable from a known .hew construct; fixtures will be added when such a construct is identified.