Client-fatal telemetry needs a bounded root-cause discriminator

Privacy-safe client telemetry should retain a deterministic, bounded root-cause discriminator in addition to error class, source, and route. Those broad fields can show recurrence and scope but cannot distinguish failures that require different remediation once raw exceptions are intentionally discarded.

Use an allowlisted error code, release or chunk identifier, or redacted normalized signature. Do not retain raw exception messages or full stacks where they may contain sensitive data. This improves diagnostic separation; it does not prove causality, identify an affected user, or replace a reproduction.