feat(admin-config-tool): task 5.4 - golden-file build determinism tests

tests/fixtures/{minimal,full}/configs/*.yaml + checked-in expected.bin
golden files, added to test_config_build.py. Catches representation
drift (bytes changing even when decoded content stays correct) that
the existing structural/decode-based assertions can't detect.

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

@@ -121,7 +121,7 @@
   - _Boundary: xcecommon.schema, xcecommon.validation_
   - _Depends: 2.1, 2.2, 1.3_
 
-- [ ] 5.4 (P) Golden-file test config build determinism
+- [x] 5.4 (P) Golden-file test config build determinism
   - Building from a minimal fixture, a full fixture covering all seven file types, and a set of intentionally invalid fixtures each produce the expected result
   - Building the same fixture twice produces byte-identical output and an identical checksum
   - _Requirements: 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 1.10, 1.11, 1.12, 9.1, 9.2, 9.3_
@@ -158,3 +158,4 @@
 - **4.2**: `deploy-ha.sh` resolves both `xceadmin` (`$SCRIPT_DIR/../xceadmin/.venv/bin/xceadmin`) and the YAML configs source (`$SCRIPT_DIR/../configs`) relative to the script's own location (`$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)`), not the caller's CWD — matches the project's other relative-path scripts and keeps the deploy working regardless of where it's invoked from. `--build-config` is parsed out of `"$@"` before positional-arg assignment so it can appear anywhere in the argument list, not just a fixed position. Verified live against the real `xceadmin` build and `src/configs/`: missing `cluster.bin` auto-builds (8.4), an existing one with no flag is left byte-for-byte untouched — same mtime, same SHA (8.5), and `--build-config` against an existing `cluster.bin` forces a rebuild that reproduces byte-identical content (8.3, confirming 9.1's determinism guarantee holds through this path too).
 - **5.1, 5.2**: Already fully satisfied by task 2.3's `test_mapping.py` and task 2.4's `test_wire.py` respectively — same requirement lists, same acceptance bullets (in-memory-only mapper coverage + the explicit two-field case for 5.1; a fixed known byte sequence + real loopback round-trips for 5.2). No new tests written; would have been pure duplication.
 - **5.3**: Found and fixed a real gap while writing `test_schema_bundling.py` — `xcecommon.validation.validate_yaml()`'s `json.loads(schema_path.read_text())` call (loading the *bundled JSON validation schema itself*, not the operator's YAML) had no error handling, so a corrupted bundled JSON schema crashed with a raw `json.JSONDecodeError` instead of a clean `ValidationError` — the same "crash vs. clean error" class of bug task 2.2 already fixed for `controller.capnp`, just on the JSON-schema side, and not caught by task 2.1 since that task's tests only exercised operator-YAML failure modes, never a corrupted *schema* file. Fixed by wrapping it and raising `ValidationError(..., constraint="schema-corrupted")`, matching the existing missing-schema handling's pattern one function above it. **Gotcha for any future task touching `xcecommon`/`xceadmin` source**: editing a source file does NOT propagate to a running `.venv` until reinstalled — always re-run `xce-build.sh xceadmin` after a source edit before trusting a bare `.venv/bin/python -m pytest` result; a stale venv (especially one left in `release`/pinned mode by a prior profile run) will silently test the *old* code and produce a false failure (or false pass) that looks like a real bug.
+- **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.