Merge ticket/015-json-field-param-metric-declaration

Resolves conflicts with ticket 018 (boolean param kind) in
src/engine.rs, webapp/src/lib/api.ts, and Params.tsx: boolean-kind
params and JSON-field-path extraction now coexist as independent
extraction modes alongside regex capture groups.

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

webapp/src/lib/api.ts

@@ -27,6 +27,8 @@ export interface ParamDeclaration {
   name: string
   message_match_regex: string
   capture_group: string
+  /** Alternative to `capture_group`: a dot-separated JSON field path (e.g. "foo.bar"), read from lines that parse as a JSON object. When set (non-empty), takes precedence over `capture_group`. */
+  json_field_path?: string | null
   kind: 'string' | 'number' | 'timestamp' | 'duration' | 'boolean'
 }
 
@@ -50,6 +52,8 @@ export type ParamRole = 'label' | 'value'
 export interface MetricParamMatcher {
   param_name: string
   capture_group: string
+  /** See `ParamDeclaration.json_field_path`. */
+  json_field_path?: string | null
   kind: 'string' | 'number' | 'timestamp' | 'duration'
   /** Defaults to 'label' server-side when omitted. */
   role?: ParamRole