# 001 — Param declarations

## Summary

Implement the `ParamDeclaration` slice of the primary index: users can declare how a
named parameter is derived from a log line (e.g. deriving a timestamp different from
the ingestion timestamp, or a log level), and the engine stores/serves those
declarations.

## Scope

- `IndexKey::ParamDeclaration { name }` and a matching `DataValue` variant for the
  declaration body (a regex against the raw log text, plus how the capture maps to a
  `MetricPrimitive` — reuse `RegexToMetricPrimitiveMatcher` / `MetricPrimitiveKind`
  from `data_value.rs`, which already exist but are unused).
- Engine message(s) to add/list/remove a param declaration.
- Validate declared regexes at declaration time (reject invalid regex / unknown
  primitive kind up front rather than failing per-log later).
- No derivation/backfill against existing logs yet — that's
  [002](002-metric-declarations.md) / [003](003-metric-bucket-derivation.md), since
  metrics are what actually get indexed into buckets. This ticket is just the
  declaration storage + validation.

## Acceptance criteria

- Declaring a param with a valid regex + capture group succeeds and is retrievable.
- Declaring a param with an invalid regex or referencing a non-existent capture group
  is rejected with a clear error, not silently stored.
- Declarations sit in their own contiguous range of the BTreeMap
  (`ParamDeclaration` between `LogParamSplit` and `ParamMetricSplit`), per the
  `IndexKey` ordering in the README.

## Depends on

None — this is foundational.