fix: scan the deck buffer, not the dashboard, in the statistics viewsHEADmain

With the default file scope, the dashboard rendered and exported from its own buffer, so every count read zero and cards.csv came out empty. The render, refresh, cycle, and CSV-export paths now run their card scans in the deck buffer the command was invoked from, stored buffer-locally, and a dead deck signals a clear user-error instead of scanning the wrong buffer.

Card links are now a registered org-drill-card link type with a follow handler, and the link path carries the file identity so RET jumps work across files. Eleven new ERT tests cover the render, refresh, export, link, and dead-buffer paths.

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