fix(runner): flush collector data on shutdown

[linkdata]

Summary

• drain pending session and histogram updates after the collector stop signal
• flush final session and histogram data before closing writer channels
• preserve the original interval start for shutdown flushes

Tests

• go test ./pkg/runner ./...

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Reviewing files that changed from the base of the PR and between b615285 and b95407b.

📒 Files selected for processing (2)

• pkg/runner/data_collector_test.go
• pkg/runner/runner.go

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[linkdata]

Additional context from comment files

These notes were split from the local markdown comment files and attached here because they describe this PR's change.

Shutdown Flush

• Bug: The collector only wrote sessions and histograms on the minute ticker; pending data could be left in memory when shutdown happened before the next tick.
• Impact: The final partial interval of session rows and well-known-domain histogram updates could be silently lost during graceful shutdown.
• Fix: After minimiser workers stop, the collector now drains queued session/update work and flushes any accumulated session and histogram data before closing writer channels.
• Reasoning: Graceful shutdown should preserve already accepted data even if the current collection interval is incomplete.
• Tests: Added collector shutdown coverage that queues a session and histogram update, stops the collector, and verifies both writer channels receive flushed data.

Shutdown Partial Interval Metadata

• Bug: Shutdown-flushed session and histogram files still derived their start time as rotationTime - 60s, even when shutdown happened partway through the current interval.
• Impact: The final partial interval could be written with filenames and aggregate metadata that overlapped the previous minute or claimed a longer collection window than the data actually covered.
• Fix: The collector now tracks session and histogram interval starts explicitly and carries those starts into flushed writer payloads; legacy callers still fall back to the previous one-minute calculation.
• Reasoning: Once shutdown can flush partial intervals, the writer contract needs both interval boundaries instead of reconstructing the start from the stop time.
• Tests: Extended collector shutdown coverage to assert flushed session and histogram payloads carry non-zero, non-inverted interval boundaries.