Make elapsed-since-last an explicit param extractor, not a Duration-kind special case (ticket 038)

Previously a Value-role param with kind: Duration silently meant "elapsed
time since the previous same-labeled event" instead of parsed text — the
same MetricPrimitiveKind::Duration variant meant two unrelated things
depending on which list (label_params vs value_param) referenced it.

Add ParamDeclaration::elapsed_since_last, a third extraction source
alongside capture_group/json_field_path: when set, the param is computed
(via a new resolve_param_value, used uniformly for label_params and
value_param) rather than text-extracted, and declare_param validates it's
paired with kind: Duration. apply_metric_to_log no longer branches on
(role, kind) — it dispatches purely on what each resolved param declares.

Migrate existing profiles: migrate_duration_value_params converts any
Duration-kind param referenced as a metric's value_param into the new
extractor, and resolve_or_create_legacy_param (ticket 037's pre-existing
migration) now carries the same meaning forward for pre-037 inline
Value-role Duration matchers.

Params.tsx offers "Elapsed since previous event" as its own Extraction
option (hides the now-irrelevant message-match-regex/capture-group
fields and forces kind to duration); Metrics.tsx labels such params
accordingly wherever they're listed as label/value param choices.

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

src/data_value.rs

@@ -22,6 +22,17 @@ pub struct ParamDeclaration {
     #[serde(default)]
     pub json_field_path: Option<String>,
     pub kind: MetricPrimitiveKind,
+    /// A third extraction source, alongside `capture_group`/`json_field_path`: when set, this
+    /// param's value is never parsed from text at all — it's computed as elapsed time since the
+    /// previous event that produced the same label set (ticket 038). Takes precedence over
+    /// `json_field_path`/`capture_group` when set, and `message_match_regex`/`capture_group`/
+    /// `json_field_path` are simply unused in that case (`declare_param` doesn't validate them for
+    /// this shape). Only meaningful paired with `kind == Duration`, since elapsed time is the only
+    /// primitive this computation can produce — `declare_param` enforces that pairing.
+    /// `#[serde(default)]` so params declared before this field existed keep parsing as plain
+    /// regex/JSON-field extraction.
+    #[serde(default)]
+    pub elapsed_since_last: bool,
 }
 
 /// Reads a value out of a JSON object by dot-separated field path (e.g. `"foo.bar"`), for
@@ -56,7 +67,11 @@ pub enum MetricPrimitiveKind {
     String,
     Number,
     Timestamp,
-    /// On a metric's Value param this is derived as elapsed time since the previous event (not parsed from text); otherwise it's a literal i64 millisecond count.
+    /// A literal i64 millisecond count. Note this is unrelated to
+    /// `ParamDeclaration::elapsed_since_last`, which is a separate, independent extraction source —
+    /// a param using that source always parses/produces `Duration`, but not every `Duration`-kind
+    /// param is computed that way (one can also just parse a literal duration out of captured text,
+    /// like any other kind).
     Duration,
     /// A standalone param declared with this kind doesn't require (or use) a capture group: its value is
     /// simply whether `message_match_regex` matches the line, case-insensitively or per the regex's own