Bootstrap seed 7/8: deep-copy bitpacked_types (9afc7d71)

Bootstrap seed — Linux x86_64 stage3 @ 9afc7d71 (chain rung 7 / 8)

This asset is the Linux x86_64 self-hosted with compiler (a stage3 build)
at commit 9afc7d717bcaedaf5f20ad95276816fef7099740.

• Commit: 9afc7d71 — Mir: deep-copy bitpacked_types in
  snapshot_sema_types (safe-free correctness)
• Role: rung 7 of 8 — the safe-free correctness fix.
• Predecessor seed: rung 6 (7d11c42e) stage3.
• What it changes: MirModule.snapshot_sema_types(&Sema) deep-copies
  bitpacked_types instead of shallow-copying the HashMap handle out of a
  borrowed &Sema. The shallow copy aliased one runtime buffer between
  last_sema and last_mir_module, which both freed at teardown → the
  double-free that NEVER_FREE_MODE=1 was masking. This is the fix that makes
  restoring real free() (rung 8) safe.
• Verification: re-verified this cycle — seeded from rung 6, full
  stage1/2/3 build, byte-identical fixpoint (stage2 == stage3). Still
  NEVER_FREE_MODE=1 at this rung (the flag flips at rung 8).

Asset: with-linux-x86_64 (+ .sha256). Use as a bootstrap seed:
WITH=./with-linux-x86_64 with build :stage1 (needs the pinned LLVM SDK in .deps/).