Add selectable declared-param columns to LogPreview (ticket 015)

Extend GET /api/logs to derive each declared param's value per log line
(regex + capture group + kind, matching that line's text) and include it as
a params map on LogEntryDto, omitting entries that don't match rather than
sending nulls. Add GET /api/params (list_params, mirroring list_metrics) so
the frontend can discover currently-declared param names. In LogPreview, add
a column-picker popover backed by that list, with the selection persisted in
localStorage under `ottyl.logPreview.visibleParamColumns`.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/engine.rs

@@ -364,6 +364,49 @@ fn param_declarations_range(
         })
 }
 
+/// Compiles every currently-declared param's regex once, so `derive_log_params`
+/// can be applied to many lines without recompiling each param's regex per
+/// line. A param whose stored regex somehow fails to compile (shouldn't
+/// happen — `declare_param` validates it before storing) is skipped rather
+/// than panicking.
+fn compiled_param_declarations(
+    index: &BTreeMap<IndexKey, DataValue>,
+) -> Vec<(String, Regex, String, MetricPrimitiveKind)> {
+    param_declarations_range(index)
+        .filter_map(|(name, message_match_regex, capture_group, kind)| {
+            Regex::new(message_match_regex)
+                .ok()
+                .map(|re| (name.to_string(), re, capture_group.to_string(), kind))
+        })
+        .collect()
+}
+
+/// Runs every declared param's regex against one log line's text, returning
+/// the derived `MetricPrimitive` values keyed by param name. A param is
+/// omitted from the result (rather than included as null/empty) when its
+/// regex doesn't match the line, its capture group doesn't match, or the
+/// captured text fails to parse as the param's declared `kind` — see
+/// `LogEntryDto::params`.
+fn derive_log_params(
+    text: &str,
+    declarations: &[(String, Regex, String, MetricPrimitiveKind)],
+) -> BTreeMap<String, MetricPrimitive> {
+    let mut params = BTreeMap::new();
+    for (name, regex, capture_group, kind) in declarations {
+        let Some(captures) = regex.captures(text) else {
+            continue;
+        };
+        let Some(raw) = captures.name(capture_group) else {
+            continue;
+        };
+        let Some(value) = parse_metric_primitive(raw.as_str(), *kind) else {
+            continue;
+        };
+        params.insert(name.clone(), value);
+    }
+    params
+}
+
 fn logs_range(index: &BTreeMap<IndexKey, DataValue>) -> impl Iterator<Item = (u64, Timestamp, &str)> {
     index.range(IndexKey::Start..IndexKey::LogParamSplit).filter_map(|(key, value)| {
         match (key, value) {
@@ -379,6 +422,17 @@ fn handle_query(index: &BTreeMap<IndexKey, DataValue>, query: EngineQuery) -> En
     match query {
         EngineQuery::Logs(params) => {
             let limit = params.limit.unwrap_or(500).min(10_000);
+            // Compile each currently-declared param's regex once up front, then
+            // apply the compiled set to every *returned* line below (ticket 015).
+            // Perf note: this is a straightforward per-returned-line,
+            // per-declared-param regex scan — O(lines_returned * declared_params)
+            // — rather than precomputing/caching derived params for the whole log
+            // range. That's fine at this codebase's current scale (v1), but if
+            // either the log volume or number of declared params grows a lot,
+            // this should move to incremental derivation at ingest time (mirroring
+            // how `apply_all_metrics` already does for metric buckets) instead of
+            // recomputing on every `/api/logs` query.
+            let compiled_params = compiled_param_declarations(index);
             let entries: Vec<LogEntryDto> = logs_range(index)
                 .filter(|(sequence, timestamp, _)| {
                     params.since_sequence.is_none_or(|since| *sequence > since)
@@ -390,6 +444,7 @@ fn handle_query(index: &BTreeMap<IndexKey, DataValue>, query: EngineQuery) -> En
                     sequence,
                     timestamp_ms: timestamp.as_millis(),
                     text: text.to_string(),
+                    params: derive_log_params(text, &compiled_params),
                 })
                 .collect();
             EngineResponse::Logs(entries)
@@ -835,4 +890,30 @@ mod tests {
         assert_eq!(bucket_values(&index, "requests", None).count(), 1);
         assert_eq!(bucket_values(&index, "durations", None).count(), 1);
     }
+
+    #[test]
+    fn logs_query_includes_declared_params_on_matching_lines_only() {
+        let mut index = BTreeMap::new();
+        declare_param(&mut index, request("level", r"level=(?P<level>\w+)", "level")).unwrap();
+        ingest_log(&mut index, 0, "level=info hello");
+        ingest_log(&mut index, 1, "no match here");
+
+        let EngineResponse::Logs(entries) =
+            handle_query(&index, EngineQuery::Logs(LogsParams {
+                since_sequence: None,
+                start_ms: None,
+                end_ms: None,
+                limit: None,
+            }))
+        else {
+            panic!("expected Logs response");
+        };
+
+        assert_eq!(entries.len(), 2);
+        assert_eq!(
+            entries[0].params.get("level"),
+            Some(&MetricPrimitive::String("info".to_string()))
+        );
+        assert!(entries[1].params.get("level").is_none());
+    }
 }