Adds `pub(crate) mod memory_profiling` (gated on the new
`memory-profiling` cargo feature) with:
- RawAlloc<MAX_FRAMES> and RawFree fixed-size POD records.
- RingBuffers<MAX_FRAMES> wrapping two `crossbeam_queue::ArrayQueue`s
  — alloc queue (default 4096 slots × 1056 B = ~4 MiB at 128 frames)
  and free queue (default 32K slots × 32 B = ~1 MiB; 8× the alloc
  slot count per design §9 because frees are pushed unconditionally
  and allocs only when sampled).
- MemoryProfileSource<MAX_FRAMES> implementing the existing Source
  trait, draining both queues each flush cycle and emitting
  AllocEvent / FreeEvent into the trace via the consolidator's
  ThreadLocalEncoder.

Drain is timestamp-ordered: at each step we emit whichever queue
head has the older timestamp. Naive 'all allocs, then all frees'
would corrupt the liveset under address reuse within a single flush
cycle (alloc → free → alloc-with-same-addr); see the
`address_reuse_within_flush_cycle_preserves_liveset` test.

Liveset tracking (HashMap<u64, LivesetEntry>) is wired in but only
activates when MemoryProfileSource is constructed with
`MemoryProfileSource::new(rings, true)`.

No allocator hook yet (later commit). Tests push synthetic RawAllocs
and RawFrees directly into the queues and verify AllocEvents and
FreeEvents come out the other end.

- Add compile-time assertion that MAX_FRAMES <= u8::MAX (frame_count field)
- Replace frames().to_vec() with direct slice of destructured array to
  avoid a heap allocation per drain in handle_alloc