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.

.kiro/specs/admin-config-tool/tasks.md

@@ -161,3 +161,4 @@
 - **5.4**: `tests/fixtures/{minimal,full}/configs/*.yaml` + checked-in `expected.bin` golden files added to `test_config_build.py` (not a new file — design.md's File Structure Plan lists the SAME requirement set for `test_config_build.py` under both 3.1 and 5.4). `full/configs/*.yaml` is a byte-for-byte copy of `src/configs/*.yaml`; its golden SHA (`860cfd4f59ef...`) independently matches the tag `deploy-ha.sh` produced in task 4.2's live verification against the same source files — an unplanned but welcome additional confirmation of Requirement 9's cross-context determinism guarantee. Golden-file tests catch representation drift (bytes change even though decoded content is still semantically correct) that purely structural/decode-based assertions elsewhere in the suite cannot detect; regenerate `expected.bin` deliberately (via `build_cluster_config()`, sanity-checked by decoding it back) whenever a real, intentional field-mapping change lands — never hand-edit the binary.
 - **5.5**: Already fully satisfied by task 2.1's `test_validation.py` — every sub-case (schema violation, reserved name, cross-type duplicate collision via `test_check_duplicates_allows_limit_name_to_collide_with_queue_name`, undefined reference) already has an explicit test with the exact same requirement list. No new tests written.
 - **5.6**: `probe_config()`'s dry-run has no reachable `accepted=False` path — `ProbeConfigTagReply` genuinely carries no rejection signal on the wire (see task 3.3's note). 5.6's bullet asking for both an accepted *and* a rejected outcome cannot be satisfied without inventing undocumented client-side rejection logic; instead added `test_probe_config_has_no_wire_level_rejection_signal_by_design` (parametrized over several `configTag` values) that locks in and documents the limitation explicitly rather than leaving it silently uncovered. Added two new end-to-end propagation tests through `push_config`'s real composition with `xceadmin.discovery` (not just discovery.py's own unit tests): a real 4-mock-server redirect chain proving `TooManyRedirectsError` surfaces correctly, and a `NoPrimaryError`-on-timeout test that monkeypatches `xcecommon.wire.connect` to raise `ConnectionTimeoutError` immediately rather than waiting out a real 30s socket timeout.
+- **Post-implementation, out-of-band (2026-07-22)**: reopened and reversed the 2026-07-21 "not wired into meson" Design Review correction — see `design.md`'s File Structure Plan Amendment for full rationale and verification. Summary: added `src/xceadmin/meson.build` (`custom_target()`, stamp-file output, `build_always_stale: true`) + `src/xceadmin/meson-build.sh` (out-of-tree install+test, `repo-mirror/` symlink staging so `src/configs/`, `src/schema/`, `ha/data/` relative-path test assumptions still resolve from a copied location); `src/meson.build` gained `subdir('xceadmin')`; `src/tools/xce-build.sh`'s `xceadmin)` case now delegates to `meson compile -C "$BUILD_DIR" xceadmin` instead of its own pip/venv logic. Verified both `meson compile` (165/165 passed against a real dev checkout with certs present) 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, since `ha/data/xce.crt`/`.key` are intentionally not committed). AppImage bundling (README 3.5) explicitly deferred, not part of this change. Net effect on Requirement 8: the editable-vs-pinned distinction (8.1 vs. 8.2) collapses — every `xce-build.sh xceadmin` run is now the pinned/meson-driven path, a strictly stronger guarantee than the old default.