Move mmap gap hints for per-guest isolation

[jserv]

Two static globals in src/syscall/mem.c, mmap_r{w,x}_gap_hint, described a specific guest's region layout and would alias if 2 guest_t instances ever coexisted in one process (test harnesses, future multi-VM use). Move both fields into guest_t.

guest_reset zeroes them; guest_{init,init_from_shm} zero them via memset. The public mmap_reset_hints() helper and its callers in src/syscall/exec.c (right after guest_reset) and src/runtime/fork-state.c (right after syscall_init in fork_ipc_recv_fd_table) are no longer needed and removed. find_free_gap loses const on its guest_t argument to mutate the per-guest hints; it is static so the change has no cross-TU API impact. The four munmap/mremap rewind sites point at g->mmap_*_gap_hint instead.

Audited the rest of the module-level state in src/. Host-derived caches (sysinfo, host_port, totalram, getloadavg, cached uname/groups/affinity/ rlimits) stay process-global by design: any future second guest in this process would share the same host stats. Other process-scoped tables (proc_table, next_guest_pid, pidfd/inotify/netlink/sysv-ipc/abstract- socket tables, futex buckets, thread_table, signal state, procemu temp dirs, log state) remain global because the current architecture is one VM per macOS process (HVF restriction) and fork uses posix_spawn into a fresh process. Documented the sysinfo cache inline as intentionally process-scoped.

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Summary by cubic

Isolate mmap gap-finder hints per guest to prevent cross-guest allocator state leaks and remove unnecessary global cleanup. This fixes aliasing in multi-guest test setups and prepares for future multi-VM scenarios.

• Refactors
  • Moved mmap_rw_gap_hint and mmap_rx_gap_hint into guest_t; zeroed in guest_reset and init paths.
  • Removed mmap_reset_hints() and its calls in exec.c and fork-state.c.
  • Changed find_free_gap to take a non-const guest_t * to update per-guest hints; remains static.
  • Updated sys_munmap/sys_mremap to rewind g->mmap_*_gap_hint instead of globals.
  • Documented sysinfo cache as intentionally process-scoped; no behavior change.

^{Written for commit f1c1a63. Summary will update on new commits.}

[cubic-dev-ai]

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