fix: Phase 3+4 review — deterministic hash, EINTR, env restore, installer wiring

- std::hash→FNV-1a (deterministic across runs, null-byte-separated)
- waitpid EINTR retry loop (prevents zombie processes under signals)
- setenv save/restore between builds (prevents cross-package env leaks)
- install::install() wired into kappa build CLI (store + DB + generations)
- Zero-padded generation filenames (gen-0001, lexicographic sort correct)
- Hardcoded paths fixed: /tmp/kappa-build→paths::temp_dir()/build
- Hardcoded destdir: /kappa/temp/destdir→paths::temp_dir()/destdir
- ensure_directories() called at main() startup
- build subcommand: -j N, --root wired end-to-end

CMakeLists.txt

@@ -33,5 +33,8 @@ add_executable(kappa
   src/tools/doctor.cpp
   src/resolve/plan.cpp
   src/fetch/fetch.cpp
+  src/build/build.cpp
+  src/sched/scheduler.cpp
+  src/install/install.cpp
 )
 target_include_directories(kappa PRIVATE include)