PayloadEnricher

Phase 34 Plan 02 (AND-PAYLOAD-01..04) — composes the payload-enrichment pipeline. Every collector is wrapped in runCatching so a single collector fault cannot fail the entire submission (T-34-02-04).

Recognized special metadata keys on the incoming FeedbackRequest:

• "__throwable__" — if present and refers to a Throwable captured elsewhere in the SDK, the formatted cause chain is appended as stack_trace. (Threadable via a side channel because FeedbackRequest carries Map<String, String>; the actual reference lives in ThrowableSidecar keyed by the same id. Kept minimally invasive — full plumbing arrives with CrashReporter in 34-04.)