Summary

• Use the existing C c_is_dir wrapper for Unix directory checks instead of shelling out
• Bootstrap path keeps the old shell command

Context

Part of the parallel build safety and performance work. Removes shell overhead
from is_dir by using the existing C binding on Unix. Neutral on fpm self-build
(compiler time dominates), but eliminates fork calls that are unsafe under
OpenMP threads.

Merge order

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:

• 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 (pending CI fix)

Verification

fpm self-build and full test suite pass:

$ fpm build --features openmp --compiler gfortran --flag '-O3 -g'
[100%] Project compiled successfully.

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

Benchmark (fpm self-build, median of 3):

parallel: 21.17s main, 21.10s branch, 0.07s faster, 0.3%
serial:   24.90s main, 24.91s branch, 0.01s slower, 0.0%

Neutral — compile time dominates; the shell overhead removal matters for
the parallel safety story, not for this benchmark.