turbo-persistence: remove Unmergeable mmap advice (#91713)
### What?
Remove the `MADV_UNMERGEABLE` (Unmergeable) mmap advice from
`turbo-persistence`'s `advise_mmap_for_persistence` helper in
`mmap_helper.rs`.
### Why?
The WSL (Windows Subsystem for Linux) kernel does not support
`MADV_UNMERGEABLE` and returns an error when it is applied, causing
turbo-persistence to fail on WSL. See:
https://x.com/von_cronen/status/2034944304712392891
Additionally, `MADV_UNMERGEABLE` is already the **default** state for
memory pages — KSM (Kernel Same-page Merging) is opt-in via
`MADV_MERGEABLE`. Explicitly setting `MADV_UNMERGEABLE` on pages that
were never marked mergeable is a no-op on standard kernels, so the call
provides no benefit while breaking WSL users.
The `MADV_DONTFORK` advice is retained, as it has a clear correctness
benefit (prevents mmap regions from being inherited by child processes
on `fork()`).
### How?
Removed the `mmap.advise(memmap2::Advice::Unmergeable)` call and its
associated doc comment from
`turbopack/crates/turbo-persistence/src/mmap_helper.rs`.
Co-authored-by: Tobias Koppers <sokra@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>
