Both meson-build.sh (venv creation) and meson-install.sh (wrapper
shebang) resolved python3 independently at run time (bare `python3 -m
venv` / `command -v python3`) -- correct as long as invoked inside
nix develop/build, but nothing prevented a stray non-nix system
python3 earlier on PATH from shadowing it.
meson.build now resolves python3 once via import('python').
find_installation('python3') -- same mechanism as find_program('go')/
capnp/packcc elsewhere in this build, printed at `meson setup` time
as "Program python3 found: YES (/nix/store/...)" -- and threads that
exact resolved path into both scripts as an explicit argument instead
of a fresh lookup.
Verified: meson setup prints the pinned python3, xceadmin target
rebuilds clean (165/165 passed), and the built venv's own python3
resolves through to that exact nix store path.
xceadmin's custom_target only built+tested (stamp-file output, no
install:true) — nix build .#xcecore succeeded but shipped nothing in
result/bin, unlike every other component.
Add meson.add_install_script('meson-install.sh'): copies the
xceadmin/xcecommon package dirs (not the whole build-dir venv, whose
pip-generated shebang hardcodes a build path that stops existing after
the build) into $out/lib/xceadmin-site, then writes a wrapper at
$out/bin/xceadmin whose shebang and PYTHONPATH are resolved against
the install script's own nix build environment — real /nix/store
paths for python3 and each dependency (pycapnp, pyyaml, jsonschema,
click), so nix's reference scanner keeps them alive in the closure.
No venv activation needed at runtime.
Verified: meson install --destdir and nix build --option sandbox
false .#xcecore both produce a working result/bin/xceadmin, runnable
standalone with a stripped-down PATH-only environment.
