Introduces a mechanism for subscribers to request different levels of
optional data emission from producers on a per-stream basis, enabling
fine-grained control over tracing overhead:

- Added stream_detail_level_t enum (NONE, BASIC, NORMAL, VERBOSE)
- Added optional xptiQuerySubscriberStreamDetailLevel subscriber callback
- Added xptiGetEffectiveStreamDetailLevel producer API
- Aggregation rule: effective level = max across all subscribers

Producers can use threshold checks to conditionally emit expensive optional
data based on the effective detail level.

Also add XPTI_HAS_STREAM_DETAIL_LEVEL macro to enable compile-time
feature detection for stream detail level functionality.

This approach allows to stay fully backward compatible: subscribers
that define new xptiQuerySubscriberStreamDetailLevel callback can work
with both old frameworks (that will ignore the callback) and new
frameworks (that will recognize the callback).

Assisted-By: Claude Sonnet 4.5