feat(admin-config-tool): wire xceadmin into meson + nix build

Reopens the 2026-07-21 design decision to keep xceadmin outside meson.
Standard onboarding consistency (README 3.3 `nix build .#xcecore` and
3.4 `meson compile` working for xceadmin like every other component)
outweighs the earlier concern about meson's single-artifact model not
fitting a multi-file pip install.

- src/xceadmin/meson.build: custom_target() wrapping install+test,
  stamp-file output, build_always_stale (matches this repo's existing
  clean-rebuild-every-invocation convention).
- src/xceadmin/meson-build.sh: out-of-tree pinned install + pytest;
  stages a repo-mirror/ symlink tree so tests' fixed-relative-path
  assumptions (src/configs/, src/schema/, ha/data/) still resolve from
  a copied location, without touching their deliberate fail-loud
  behavior.
- src/meson.build: subdir('xceadmin') under component != 'xcecli'.
- src/tools/xce-build.sh: xceadmin case now delegates to
  `meson compile -C "$BUILD_DIR" xceadmin` instead of its own
  pip/venv implementation, like every other component.

Verified both build paths: meson compile (165/165 passed) and
nix build --option sandbox false .#xcecore_debug (128 passed / 37
skipped — cert-dependent tests correctly skip under nix's
git-tracked-only hermetic source). AppImage bundling (README 3.5)
is explicitly deferred to future work, not part of this change.

src/xceadmin/pyproject.toml

@@ -11,9 +11,11 @@ dependencies = [
     "click>=8.0",
 ]
 
-# NOT wired into meson (src/meson.build / src/meson.options) — xceadmin is
-# dispatched by src/tools/xce-build.sh's case-statement pattern instead.
-# See .kiro/specs/admin-config-tool/design.md, File Structure Plan.
+# Wired into meson (src/xceadmin/meson.build custom_target, pulled in by
+# src/meson.build's subdir('xceadmin')) so `nix build .#xcecore[_debug]`
+# and `meson compile` install + test this package like every other
+# component; src/tools/xce-build.sh's xceadmin case now just delegates to
+# that same meson target.
 
 [project.optional-dependencies]
 test = ["pytest"]