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Debug-allocator fixture lane

These fixtures give the test floor eyes for the over/under-drop blind spot it is structurally unable to see (the floor has zero Vec[Drop]). Each is run under the native debug allocator and checked against its //! expect-debug-alloc: directive. A fixture may also set //! debug-alloc-filter: all|non-root|roots to select the leak view for that test. Run the lane with:

with build :debug-alloc-tests

(which builds tools/debug_drop.w and runs it in check mode over this corpus).

The directives are now soundness gates for #607's inline-drop field ownership work. Inline-drop fields must be freed exactly once in every covered construction and escape shape: leak count=0, never DOUBLE FREE.

  • The da_vecdrop_* fixtures cover inline Vec[Drop] fields across in-place construction, local move-in, rvalue move-in, nested structs, tail/trailing positions, field-receiver push tails, and field chaining. They all expect leak count=0.
  • da_manual_double_free (with_free twice) expects DOUBLE FREE — a stable, compiler-independent check that the ledger detects a double free.
  • da_drop_origin_double_free duplicates a Vec[Drop] header and explicitly drops both values; it expects first_drop=drop# so generated drop tags stay wired through MIR, codegen, and the allocator ledger.
  • da_root_filter marks a deliberately leaked allocation as a process-lifetime root and runs with debug-alloc-filter: non-root; it expects leak count=0.
  • da_pod_vec expects leak count=1 — a POD Vec[i32] buffer is not freed under the narrow drop gate (#608, separate from #607; documented here so the instrument's view of it is pinned).

See docs/debug-allocator.md for the design and tools/debug_drop*.lldb for resolving the source sites behind a flagged address.