fix: serialize mkdir shell fork on the run_command lock

[krystophny]

Summary

• Move directory creation in build_target from an unnamed !$omp critical
  to the named critical(run_command) lock that guards all other shell forks
• mkdir shells out twice (is_dir runs test -d, then mkdir -p), both of
  which fork; the unnamed critical was a different lock than run_command, so a
  mkdir fork on one thread could run concurrently with a compile/link/archive
  shell fallback on another — the exact concurrent-fork hazard the
  run_command lock exists to prevent

Merge order

Independent — no ordering constraint. Logically related to the argv spawn chain
but does not depend on it.

Note: textual overlap with #1289 (interface-aware rebuild) — both edit the
!$omp critical block in build_target. Whichever merges second rebases.

Independent PRs (any order):

• Native is_dir (perf: use native directory checks on Unix #1291)
• Native mkdir (perf: create Unix directories without shell #1292)
• Build only selected run targets (perf: build only selected run targets #1293)
• Skip dry-run for run/test --list (perf: skip dry-run build for run --list and test --list #1294)
• Skip dry-run for build --list (perf: skip dry-run build for build --list #1295)
• Serialize pretty-mode progress under OpenMP (fix: serialize pretty-mode build progress output under OpenMP #1296)
• Dependency-driven build scheduler (feat: dependency-driven build scheduler (no per-region barrier) #1297)

Stacked chain (merge in order):

1. Fortran compiles via argv (feat: exec Fortran compiles via argv (posix_spawn/CreateProcess) #1299)
2. Parallel link/archive via argv (feat: run link and archive in parallel via argv spawn #1300)
3. C/C++ compiles via argv (feat: exec C/C++ compiles via argv, close the shell-fork gap #1301)

Fork-safety fix:

• This PR — Serialize mkdir on run_command lock (fix: serialize mkdir shell fork on the run_command lock #1298)

After the above:

• Interface-aware rebuild (Skip recompiling dependents when a module interface is unchanged (gfortran) #1289, already open)
• Enable default features via manifest (feat: auto-apply features marked default = true in the manifest #1302)

Refs: #962

Verification

$ fpm build --flag -fopenmp
[100%] Project compiled successfully.

$ fpm test --flag -fopenmp
TALLY;TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
PASSED: all 35 tests passed