config: generalize config_validate into a shared schema engine

require_schema(config, schema) is a generic recursive declarative
checker (nested-dict schema, leaves name required type(s), each
intermediate object node gets its own dict-type check so a wrong-typed
section is named directly instead of surfacing as a confusing "missing
child" error). validate_temp_ctrl() is now just require_schema(config,
TEMPCTRL_SCHEMA) against a declarative schema dict, replacing the
hand-rolled nested loops.

utils/sud_validate.py's static structural check ('steps' must be a
list) now goes through the same require_schema()/SUD_SCHEMA - one
shared mechanism for both server config.json and sud.json documents.
The genuinely dynamic per-step check (ramp.rate required only when a
step sets its own temperature) stays hand-written, since it depends on
the document's content, not just its shape.

Added TestRequireSchema, exercising the generic engine directly
against a schema unrelated to TempCtrl.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F4oEE99rkKVU8L8tkzXWdG

utils/sud_validate.py

@@ -1,6 +1,18 @@
-from utils.config_validate import ConfigError, require
+from utils.config_validate import ConfigError, require, require_schema
 from components.sud import Sud
 
+# The only part of a sud.json document's shape that's fixed regardless of
+# content - same declarative mechanism validate_temp_ctrl() uses for
+# config.json's TempCtrl section (see config_validate.py's require_schema()).
+# Everything else about a schedule (how many steps, which fields each one
+# overrides vs. inherits from default.step, whether a step needs ramp.rate
+# at all) depends on the document's own content, not just its shape, so it
+# can't be expressed as a static schema - handled by the step loop below
+# instead, still built on the same require()/ConfigError primitives.
+SUD_SCHEMA = {
+    'steps': list,
+}
+
 
 def validate_sud(data):
     """Fail-fast structural check for a sud.json document - same rationale
@@ -15,7 +27,7 @@ def validate_sud(data):
     Reuses Sud._parse_data() itself (rather than re-implementing default-
     merging/step-building) so what's checked here can never drift from what
     actually gets parsed at runtime."""
-    require(data, 'steps', type=list)
+    require_schema(data, SUD_SCHEMA)
     if not data['steps']:
         raise ConfigError("'steps' must be a non-empty list")