turbo-tasks: mimalloc-aware grow/shrink for LazyTail
mimalloc rounds allocation requests up to bin boundaries — asking for 65 B
reserves the 80 B bin, so the next push up to 80 B is free. Two new
behaviors:
- `TurboMalloc::good_size(n)`: thin wrapper over `mi_good_size` (falls
back to `next_power_of_two().max(16)` when `custom_allocator` is off).
- `LazyTail::grow_to` allocates `good_size(min_bytes)` instead of the
previous power-of-two doubling. Each grow now reserves exactly the bin
it would have ended up in anyway, eliminating the second realloc that
used to fire when an earlier push left the buffer just below the bin
boundary.
- `LazyTail::shrink_to_fit` is bin-aware: it only reallocates when
`good_size(len) < good_size(cap)`. Within a bin the realloc is free
but pointless, so we skip it.
50 backend tests pass; whole workspace builds.
