Add tickets to improve metric abstractions

tickets/039-metric-param-query-resolves-declared-params.md

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+# 039 — `metric[param]` query syntax should resolve against declared params
+
+## Summary
+
+`compile_metric_comparison` (`src/engine.rs`) resolves the `param` in
+`metric[param] <op> value` by reading whatever key happens to be present in a
+bucket's `labels` map at query time. There's no link back to a metric's
+actual declared label params, so a typo or a param that was never a label on
+that metric fails silently (empty match) instead of being rejected up front.
+
+## Scope
+
+- Once [037](037-unify-param-declarations.md) lands, validate `metric[param]`
+  filter expressions against the target metric's declared `label_params` at
+  parse/compile time.
+- Return a clear filter-validation error when `param` is not one of the
+  metric's label params, instead of silently matching nothing.
+- Confirm `src/filter_parser.rs` and `compile_metric_comparison` share the
+  same notion of "param" as the Params panel and metric declarations after
+  this change (no separate resolution path).
+
+## Acceptance criteria
+
+- `metric[nonexistent_param] == "x"` is rejected with a validation error
+  referencing the metric's actual label params, rather than silently
+  returning no matches.
+- Existing valid filter queries using real label params continue to work
+  unchanged.
+
+## Depends on
+
+[037](037-unify-param-declarations.md).