ml_log: flush the store on JsonLogger.sync()/close()

[derek-tml]

Stack from ghstack (oldest at bottom):

• trace + trainers: route tracing, Gantt, logtree, and log_path through Storage #762
• recipes: route run-artifact I/O through Storage #761
• cli_utils: make check_log_dir cloud-safe #760
• checkpoint_utils: read and write checkpoints through Storage #759
• -> ml_log: flush the store on JsonLogger.sync()/close() #758
• logtree: add Trace.to_html() #757
• stores: add exists_tree/rmtree, mkdir flag, normalize_log_path, and store HTML/flush helpers #756

On cloud backends, appended files (metrics.jsonl, checkpoints.jsonl,
timing_spans.jsonl) are staged locally and only uploaded on flush. Wire
sync()/close() to TrainingRunStore.flush() so metrics are not silently
lost at shutdown.

Co-authored-by: Cursor cursoragent@cursor.com

[derek-tml]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

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