turbo-tasks-malloc: report memory from mimalloc (#97761)
### What?
`turbo-tasks-malloc` keeps a process-wide `ALLOCATED` atomic to answer
`memory_usage()`. To optimize performance thread local buffers were
maintained but this lead to code size issues. So instead we rely on
mimalloc apis saving 1.6M of binary size
### Why not ask the OS?
That was the obvious alternative, and it does not work well. The natural
per-platform figures are not the same quantity: macOS `phys_footprint`
and Windows `PrivateUsage` account for compressed and swapped pages, but
Linux `VmRSS` does not — so under swap or zram the Linux number can read
flat, or fall, while real consumption climbs. Closing that gap means
`/proc/self/smaps_rollup` (`Pss + SwapPss`), which is not the cheap
one-line read it looks like, it takes on the order of [100ms per
4GB](https://github.com/ncabatoff/process-exporter/issues/246), and it
would contend with other allocations in our process.
### How?
`memory_usage()` reads `current_commit` from `mi_process_info`. We fall
back to the `ALLOCATED` atomic only when `custom_allocator` is off.
### Behaviour changes
**It does not track frees in lock step.** mimalloc reuses and purges
pages on its own schedule or when eviction calls `collect`
### Size
Rust inlines `#[global_allocator]` methods into every allocation site,
so the buffer arithmetic and the branch guarding it were duplicated
across the whole binary. Dropping them from the default build is where
the win comes from.
`libnext_napi_bindings.dylib`, release profile as shipped (thin LTO,
`codegen-units = 1`).
| Section | canary | this PR | Δ |
|---|---:|---:|---:|
| `__text` | 64,833,760 | 63,319,776 | **−1,513,984** |
| `__eh_frame` | 8,966,904 | 8,748,464 | −218,440 |
| `__gcc_except_tab` | 2,234,428 | 2,229,484 | −4,944 |
| `__unwind_info` | 1,489,072 | 1,484,644 | −4,428 |
| **Total** | | | **−1,741,796 = −1.66 MiB** |
### Performance
`cargo bench -p turbo-tasks-malloc --bench allocation`, macOS aarch64,
revisions interleaved across 3 rounds so drift hits both arms equally.
Figures are the paired per-round deltas.
| benchmark | canary | this PR | mean Δ | per-round |
|---|---:|---:|---:|---|
| `alloc_dealloc` | 6.65 ns | 6.45 ns | **−2.9%** | −3.7% .. −2.4% |
| `alloc_realloc_dealloc` | 18.62 ns | 16.76 ns | **−10.0%** | −10.2% ..
−9.7% |
Removing the counter takes the buffer arithmetic and its guarding branch
out of every allocation, and a realloc pays that twice — which is
roughly the shape of the result. That said, the absolute numbers are
small (a couple of hundred picoseconds on `alloc_dealloc`) and this
benchmark is noisy, so treat the direction as the signal rather than the
magnitude.
