fix(hy-emit,hy-check): fix O8 slot-loan rebind/branch races and A3/T24 soundness gaps (#603)

[sonson0910]

Summary

Closes three related P0 soundness holes in O8 slot-loan lowering (hy-emit) and A3/T24 slot-loan checking (hy-check):

1. hy-emit (compiler/crates/hy-emit/src/stmt.rs, expr.rs, lib.rs):

   • Stop remaining_stmts_use_loan when an unshadowed Stmt::Assign (AssignOp::Assign) overwrites PlaceKind::Ident(name) (after checking RHS values and non-ident target sub-expressions), and delegate block_mentions_ident to remaining_stmts_use_loan so a plain-value or slot-loan rebind inside a block does not falsely keep the old loan alive past move m.
   • Hand off active slot loans into Stmt::If, Stmt::Block, and Stmt::Managed when the construct rebinds the loan variable or moves its base map, and update flush_loan_for_name to return the frame index so rebinding an outer slot loan to a new slot loan updates the loan in place after flushing the old slot.
   • Save and restore branch_inherit in emit_fn_lit (expr.rs) so closures evaluated inside an if condition do not consume the enclosing if statement's branch_inherit handoff.
   • Save and restore pointer_vars and shadowed_since_loan in save_scope / restore_scope (lib.rs) so inner-block shadowing does not leak into outer slot-loan flushes.

2. hy-check (compiler/crates/hy-check/src/loans.rs):

   • Extend a3_path_is_prefix_or_eq so rebinding an enclosing struct container (h = holder{...}) while a slot loan on a nested field map (&h.m[k]) is live is rejected with HYB-E3102.
   • Invoke check_a3_method_or_inline_call from Resolved::Method and Resolved::None in process_call so user-defined methods (h.drop_a(), b.put(m)) and immediately-invoked closure literals ((fn(x) { delete(x, "a") })(m)) that mutate a map during a live slot loan are rejected with HYB-E3102.

3. hy-check (compiler/crates/hy-check/src/addr_loan.rs):

   • Extend is_slot_loan_expr to recognize &s.field (Expr::Field) slot loans alongside &m[k] (Expr::Index), clear slot_loan on multi-return reassignments (n, ok = f()), and reject &n.arr[0] (Expr::Index projections rooted at a slot loan) with HYB-E3104 (T24).

Verification

• cargo test --workspace --quiet
• python3 tools/check_test_registration.py
• python3 tools/check_conflict_markers.py
• End-to-end hyb check, hyb generate, and CGO_ENABLED=1 CC=gcc go run -race . on reproducers

Fixes #603