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/createDebouncedState/createDebouncedState.svelte.ts

@@ -1,5 +1,11 @@
 import { debounce } from '$shared/lib/utils';
 
+/**
+ * Default debounce delay used when no wait is provided. Picked to feel
+ * snappy for typing while still coalescing API-bound side effects.
+ */
+export const DEFAULT_DEBOUNCE_MS = 300;
+
 /**
  * Creates reactive state with immediate and debounced values.
  *
@@ -23,7 +29,7 @@ import { debounce } from '$shared/lib/utils';
  * <p>Searching: {search.debounced}</p>
  * ```
  */
-export function createDebouncedState<T>(initialValue: T, wait: number = 300) {
+export function createDebouncedState<T>(initialValue: T, wait: number = DEFAULT_DEBOUNCE_MS) {
     let immediate = $state(initialValue);
     let debounced = $state(initialValue);