Add gauge/counter semantics to metric declarations (ticket 097)

Metrics previously only declared a data type (String/Number/...), so
rate() could be applied to a point-in-time gauge reading and sum()/avg()
could be applied directly to a cumulative counter reading with no guard
against either. Adds a MetricSemantics enum (Gauge, default; Counter)
threaded through DeclareMetricRequest, the engine's MetricDeclaration
data value, profile persistence, the API/MCP declaration routes, and the
Metrics panel's declaration form, then uses it in evaluate_promql to
reject rate() on a declared gauge and bare sum()/avg() on a declared
counter (suggesting rate() first). The guard only applies to metrics
with a captured value_param — bare occurrence-count metrics are always
safe under both operations, so existing dashboards are unaffected.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TeyhRfMYjMditek6geDRoQ

src/engine/snapshot/query.rs

@@ -280,6 +280,7 @@ pub(super) fn handle_query_inner(
                         value_param,
                         matched_labels,
                         unmatched_labels,
+                        semantics,
                     )| {
                         MetricDeclarationDto {
                             name: name.to_string(),
@@ -288,6 +289,7 @@ pub(super) fn handle_query_inner(
                             value_param: value_param.map(|s| s.to_string()),
                             matched_labels: matched_labels.cloned(),
                             unmatched_labels: unmatched_labels.cloned(),
+                            semantics,
                         }
                     },
                 )