# 038 — Make Duration an explicit derived-param kind, not a hidden case of Value

## Summary

Today a `Value`-role param with `kind: Duration` silently means something
different from every other kind: instead of parsing extracted text, it means
"elapsed time since the previous same-labeled event"
(`apply_metric_to_log`, `src/engine.rs`, backed by a `LastEventTimestamp`
index entry). Same enum variant (`MetricPrimitiveKind::Duration`), two
unrelated meanings depending on role — one is "parse this text as a
duration," which doesn't apply here at all; the other is "compute elapsed
time from event sequencing." This is a silent special case a reader has to
already know about to understand the code.

## Scope

- Add an explicit extractor variant, e.g. `Extractor::ElapsedSinceLast {
  grouped_by: Vec<String> }`, distinct from `Regex`/`JsonPath`.
- A param declared with this extractor is computed (elapsed time since the
  previous event sharing the given label set), never text-extracted.
  `MetricPrimitiveKind` no longer needs a role-dependent special case for
  `Duration`.
- Update `apply_metric_to_log` to branch on extractor kind instead of on
  `(role, kind)` combination.
- Update the Params panel / metric declaration UI (`Params.tsx`,
  `Metrics.tsx`) to offer "elapsed since previous event" as its own
  extractor option rather than a regex+kind combination that happens to mean
  something else.
- Migration: existing declared/embedded params using `Value` role +
  `Duration` kind convert to the new extractor variant.

## Acceptance criteria

- A "duration" param's behavior (elapsed time since last matching event) is
  selectable directly in the UI without going through a regex field that is
  never actually used.
- `apply_metric_to_log` has no branch that changes meaning based on role for
  a given kind.
- Existing profiles using the old Value+Duration combination behave
  identically after migration.

## Depends on

[037](037-unify-param-declarations.md).