JSON streamer: emit valid, correctly-nested event objects

[zhaozhiwen]

Reworks the JSON event serialization so events are valid, correctly-nested JSON. Two coupled defects, fixed together because both live in the shared brace-accounting of endEventImpl:

#116 — header never closed. startEventImpl opens "header": { but nothing closed it, so generated and detectors were emitted inside header instead of as siblings. (Single-detector output still parsed, but with the wrong shape.) Fixed by closing the header right after its fields are written (and in the endEventImpl fallback when the header step is skipped), and dropping the now-redundant header-closing brace from endEventImpl.

#115 — multi-detector digitized produced invalid JSON. Digitized data was written inline and "digitized_by_detector" was closed on every per-detector call. The base class interleaves publishEventTrueInfoData/publishEventDigitizedData per detector (gstreamer.cc:93-111), so a second digitized detector extended an already-closed object — extra braces, misplaced keys. (Note: the per-issue proposed fix of "open once, close in endEvent" does not hold under this interleaving — a later detector's true-info would land inside the still-open digitized object.) Fixed by buffering each detector's digitized array and emitting them together as one well-formed "digitized_by_detector" object in endEventImpl.

The existing schema is preserved: per-detector "digitized": [] placeholder plus the "digitized_by_detector" map.

Validation: ran the gstreamer JSON example extended to two detectors (ctof + ftof) in the Geant4 11.4.1 / Qt6 dev container. Before: json.tool rejects the output (Expecting ',' delimiter). After: all worker files parse as valid JSON, with header/generated/detectors as siblings and both detectors' 10 digitized hits present under digitized_by_detector.

Note: filed as one PR because #115 and #116 both rewrite the same endEventImpl brace logic and cannot be cleanly separated.

Fixes #115
Fixes #116