Add Boolean param kind that is always present true/false (ticket 018)

MetricPrimitiveKind gains Boolean, serialized as "boolean". A param
declared with kind Boolean skips capture-group validation and, instead
of being omitted when message_match_regex doesn't match, is present on
every log line as true/false based purely on whether the regex matches
(case-insensitively or per the regex's own flags). Params.tsx lets you
pick boolean as a kind and disables the now-unused capture group input.

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

src/data_value.rs

@@ -33,6 +33,11 @@ pub enum MetricPrimitiveKind {
     Timestamp,
     /// On a Value-role param this is derived as elapsed time since the previous event (not parsed from text); otherwise it's a literal i64 millisecond count.
     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
+    /// flags — see `crate::engine::derive_log_params`. Unlike other kinds it's present as `true`/`false`
+    /// on every line rather than omitted when unmatched.
+    Boolean,
 }
 
 /// Value type paired with [`crate::index_key::IndexKey`] in the engine's primary index.