Add provider-level progress, heartbeats, and clean interrupt handling to source loading

[anth-volk]

During a full pe_us_data_rebuild_checkpoint pipeline build after merging origin/main into codex/fix-146-narrow-lazy-imports, the build reached 02_source_loading and then remained there for roughly 2h55m without any provider-level progress or manifest heartbeat.

Observed state:

• 01_run_profile completed.
• 02_source_loading started at 2026-06-03T15:49:38Z.
• The build never reached 03_source_planning.
• stage_artifacts/manifests/02_source_loading.json still showed status: running.
• updatedAt remained equal to startedAt.
• No completed outputs were present.
• Required outputs were still missing:
  • observation_frame_summary
  • source_descriptors
  • source_relationships
• After manual termination, the manifest remained in running state with no failure/interruption reason.

This means source loading is currently difficult to diagnose: after a long runtime, we cannot tell whether it is making expected progress, stuck on a specific provider, retrying a cache/download path, or spending time in a pathological slow path.

Recommended fix:

1. Add provider-level source-loading progress events, at least:
   • provider started
   • provider completed
   • provider failed
   • elapsed time
   • row/entity counts where available
   • cache/download paths where relevant
2. Heartbeat 02_source_loading.json periodically and after each provider, including the current provider and last successful provider.
3. Persist partial per-provider summaries so reruns are diagnosable without restarting blind.
4. Catch SIGTERM/KeyboardInterrupt in the stage runtime or stage writer and mark the active stage as failed/interrupted with timestamp and reason, instead of leaving it as running.
5. Add unit tests for heartbeat updates and interrupted-stage failure recording.

Notably, this was not a Python traceback or obvious missing dependency. The first blocker was source-loading observability and clean failure recording during a long-running full build.