feat: add remix mode for spark table rolling

Introduces a `remix` option that allows each data column in a spark
table to be rolled independently. This is enabled via a `remix=true`
attribute in the directive's extra attributes.

Also refactors spark table scanning by moving markdown parsing logic
from the frontend to a new CLI-side `spark-scanner.ts` utility,
improving separation of concerns.

src/components/journal/JournalInput.tsx

@@ -128,7 +128,8 @@ export const JournalInput: Component = () => {
         const key = (parsed.payload as { key: string }).key;
         const match = comp.data.sparkTables.find((s) => s.slug === key);
         const csvPath = match?.csvPath ?? "";
-        const p = await resolveSparkPayload({ key, csvPath });
+        const remix = match?.remix ?? false;
+        const p = await resolveSparkPayload({ key, csvPath, remix });
         const result = sendMessage("spark", p);
         const r = unwrap(result);
         finish(r.ok, r.err);