fix(analytics): replace NUL-byte map-key separators with a visible one

Two in-memory key builders joined their parts with a literal NUL
(`${a}\x00${b}`): the sleep writer-bucket key and the correlation
pair key. The embedded NUL made grep classify both files as binary, so a
plain `grep symbol src/**` skipped them silently.

Swap the separator for U+241F (␟, the unit separator). It cannot appear
in a `MeasurementSource` enum literal, a device-type tag, or a
correlation-channel name, so the key semantics are identical — the sleep
key still slices its source prefix unambiguously and the correlation Set
still excludes double-counts. Both keys are ephemeral (one pass, never
persisted), so the byte change is invisible to any stored state.

src/lib/analytics/sleep-night.ts

@@ -309,10 +309,11 @@ const RECONSTRUCTED_TIMELINE_SOURCES: ReadonlySet<string> = new Set<string>([
 
 /**
  * Separator between the source and device-type parts of a writer key.
- * `` cannot appear in a `MeasurementSource` enum literal or a
- * device-type tag, so the split-back is unambiguous.
+ * The unit-separator character (U+241F) cannot appear in a
+ * `MeasurementSource` enum literal or a device-type tag, so the
+ * split-back is unambiguous.
  */
-const WRITER_KEY_SEP = "";
+const WRITER_KEY_SEP = "\u241F";
 
 /**
  * Writer-bucket key of a stage row: the source, refined by the device-type

src/lib/insights/correlation-discovery.ts

@@ -596,9 +596,17 @@ export function filterSeriesToWindow(
   }));
 }
 
+/**
+ * Separator joining the two channel names into a stable pair key. The
+ * unit-separator character (U+241F) cannot appear in a correlation-channel
+ * name, so two distinct pairs never collide. The key is used only for Set
+ * membership (never split back), and lives for one discovery pass.
+ */
+const PAIR_KEY_SEP = "\u241F";
+
 /** Stable pair identity for double-count exclusion. */
 function pairKey(behaviour: string, outcome: string): string {
-  return `${behaviour}${outcome}`;
+  return `${behaviour}${PAIR_KEY_SEP}${outcome}`;
 }
 
 export interface EmergingCorrelationResult {