# 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).