feat: add NVTX tracing for framework pins and source writes

[aknvda]

Remote delivery currently gives little visibility into framework pin waits or native write ownership. Add optional KVCR-domain NVTX annotations for the pin callback, asynchronous pin/waiter lifecycle, NIXL posting, first native DONE, cancellation, handle release, and unresolved shutdown. Static event names carry fresh structured payloads; logical failure remains distinct from native completion and safe release.

Tracing defaults to low when the optional profiling dependencies are installed. KVCR_NVTX_LEVEL=off|low|medium controls payload work without checking profiler attachment. The schema keeps signed operation handles, source-operation and target-incarnation identities, explicit unknown fields, and bounded UTF-8 context mapping. Target/hint/caller hooks are a follow-up.

Validation: 183 focused Linux tests passed on the source branch; real pin capture example and profiling documentation included. The combined source/target follow-up has separate two-process DRAM/VRAM capture evidence. An independent review found a lost route-refusal cause; the added regression failed before the fix and passes afterward. Commit sign-offs included.

Known review item: a source that selects zero descriptors reports PARTIAL with an explicit zero count; the target/caller reports FAILED. This does not change transfer or memory ownership. Framework utilization, session and parent-session IDs are unavailable through current bindings and are not inferred. Full public-fetch fan-out is outside this contribution.