refactor: extract magic constants — wave 5 (single-site thresholds)

Long-tail cleanup: threshold and default-value literals in shared
helpers get named module-level constants.

- CharacterComparisonEngine: CHAR_PROXIMITY_RANGE_PCT (5),
  DEFAULT_RENDER_SIZE_PX (16) — kept local instead of importing
  DEFAULT_FONT_SIZE from \$entities/Font because \$shared/lib cannot
  legally upward-import from \$entities per FSD (also avoids an init
  cycle through the mocks barrel).
- typographySettingsStore: BASE_SIZE_EPSILON (0.01) — rounding-jitter
  guard for baseSize reconciliation.
- createDebouncedState: DEFAULT_DEBOUNCE_MS (300) — exported so callers
  can mirror the default.
- createVirtualizer: MEASUREMENT_EPSILON_PX (0.5) — minimum height
  delta before committing a re-measured row.
- createPerspectiveManager: PERSPECTIVE_TOGGLE_THRESHOLD (0.5) — the
  halfway point on the 0-1 spring that flips isBack/isFront.

Skipped #19 (PerspectivePlan defaults) per review — marginal gain.

src/shared/lib/helpers/createVirtualizer/createVirtualizer.svelte.ts

@@ -4,6 +4,12 @@
  * Used to render visible items with absolute positioning based on computed offsets.
  */
 
+/**
+ * Minimum height delta (in px) required to commit a re-measured row height.
+ * Sub-pixel diffs are treated as measurement noise to avoid spurious re-flows.
+ */
+const MEASUREMENT_EPSILON_PX = 0.5;
+
 export interface VirtualItem {
     /**
      * Index of the item in the data array
@@ -381,8 +387,8 @@ export function createVirtualizer<T>(
                     if (!isNaN(index)) {
                         const oldHeight = measuredSizes[index];
 
-                        // Only update if the height difference is significant (> 0.5px)
-                        if (oldHeight === undefined || Math.abs(oldHeight - height) > 0.5) {
+                        // Only update if the height difference is significant
+                        if (oldHeight === undefined || Math.abs(oldHeight - height) > MEASUREMENT_EPSILON_PX) {
                             measurementBuffer[index] = height;
                         }
                     }