feat(xceadmin): validate percent-based limit values are in [0.0, 1.0] (bug-1833)

Adds an if/then JSON Schema (draft 2020-12) conditional constraint to
limits.schema.json's Limit definition: when percent is true, value
must be a number in [0.0, 1.0]. This is the class of authoring mistake
bug-1831/bug-1832 fixed manually (night_mem_pct: value: 50 instead of
0.5) -- now caught automatically at real `xceadmin config build` time
(the actual cluster.bin generation path deploy-ha.sh uses), not merely
by a unit test.

Empirically verified against the real jsonschema library this repo
uses: a violation surfaces with the exact same file/json_pointer/
constraint contract validate_yaml()'s existing ValidationError already
provides, so zero changes were needed to its error-reporting code.

While implementing this, the new constraint caught an identical
pre-existing defect in xceadmin's own checked-in "full" golden test
fixture (fixtures/full/configs/limits.yaml, night_mem_pct: value: 50)
-- fixed the same way, and regenerated the checked-in expected.bin
golden artifact to match the corrected input.

Added 3 tests to test_validation.py: reject an out-of-range percent
value, accept an in-range one, and confirm a non-percent value is
never range-checked regardless of magnitude.

Build gotcha discovered and documented (cerebrum.md): this repo has
two copies of every bundled schema -- src/configs/schema/ (canonical)
and src/xceadmin/src/xcecommon/schema_files/json/ (a gitignored local
mirror meson-build.sh always overwrites from the canonical copy on
every build). Only the canonical copy needed a source-controlled edit.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLSNJhNoULvVcUhXEFafQC

src/xceadmin/tests/fixtures/full/configs/limits.yaml

@@ -19,13 +19,15 @@ limits:
     value: 4
     per_user: true
 
-    # queue-scoped memory cap, percentage of total mem resource
+    # queue-scoped memory cap, percentage of currently-available mem resource
+    # (bug-1831/1832: percent-based value is a 0-1 fraction, not a 0-100
+    # number -- 0.5 = 50%, matching src/configs/limits.yaml's own fix)
   - name: night_mem_pct
     queues:
       members:
         - night
     resname: mem
-    value: 50
+    value: 0.5
     percent: true
 
     # host-scoped cap, applied per host in hg1