Expose whether a call started from a channel update

[lilhoser]

Closes #1152

Summary

• expose the existing grant/update start provenance to call-end plugins as started_from_update
• include the same factual boolean in call JSON
• preserve the existing message classification and add focused GRANT/UPDATE coverage

Rationale

Trunk Recorder already records whether Call_impl was created from a channel grant update rather than the original grant. That fact is used by the existing Call was UPDATE not GRANT log, but plugins and call JSON consumers cannot currently observe it.

This change exposes the existing state without claiming that audio is truncated or incomplete. The ordered transmission list already contains the first observed transmission timestamp, so no redundant timestamp is added.

Plugin compatibility

The new Call accessor is appended to the interface, and started_from_update is appended as the final Call_Data_t member so existing member offsets are preserved. Call_Data_t is passed by value across the C++ plugin boundary, however, so its changed size still requires core and plugins to be rebuilt from a compatible source pair.

Validation

• full Trunk Recorder build, including bundled plugins and an external call-end plugin: passed
• CTest: 2/2 passed (call_started_from_update_tests and the external plugin provenance/audio test)
• staged plugin installation rewrote its build-tree RUNPATH to /usr/local/lib/trunk-recorder