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/CharacterComparisonEngine/CharacterComparisonEngine.svelte.ts

@@ -4,6 +4,20 @@ import {
     prepareWithSegments,
 } from '@chenglou/pretext';
 
+/**
+ * Width of the character morph "halo" around the slider thumb, in percent
+ * of container width. Characters within this window get partial blending
+ * instead of a hard A→B flip.
+ */
+const CHAR_PROXIMITY_RANGE_PCT = 5;
+
+/**
+ * Default render size in px when callers omit the `size` arg on `layout()`.
+ * Kept as a local constant to avoid pulling `$entities/Font` into
+ * `$shared/lib` (would create an FSD-illegal upward import cycle).
+ */
+const DEFAULT_RENDER_SIZE_PX = 16;
+
 /**
  * A single laid-out line produced by dual-font comparison layout.
  *
@@ -129,7 +143,7 @@ export class CharacterComparisonEngine {
         width: number,
         lineHeight: number,
         spacing: number = 0,
-        size: number = 16,
+        size: number = DEFAULT_RENDER_SIZE_PX,
     ): ComparisonResult {
         if (!text) {
             return { lines: [], totalHeight: 0 };
@@ -260,7 +274,7 @@ export class CharacterComparisonEngine {
         const chars = line.chars;
         const n = chars.length;
         const sliderX = (sliderPos / 100) * containerWidth;
-        const range = 5;
+        const range = CHAR_PROXIMITY_RANGE_PCT;
         // Prefix sums of widthA (left chars will be past → use widthA).
         // Suffix sums of widthB (right chars will not be past → use widthB).
         // This lets us compute, for each char i, what the total line width and
@@ -291,6 +305,7 @@ export class CharacterComparisonEngine {
         const totalRendered = chars.reduce((s, c, i) => s + (isPastArr[i] ? c.widthA : c.widthB), 0);
         const xOffset = (containerWidth - totalRendered) / 2;
         let currentX = xOffset;
+
         return chars.map((char, i) => {
             const isPast = isPastArr[i] === 1;
             const charWidth = isPast ? char.widthA : char.widthB;

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);
 

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

@@ -34,6 +34,13 @@ import { Spring } from 'svelte/motion';
  */
 const PERSPECTIVE_SPRING_CONFIG = { stiffness: 0.2, damping: 0.8 } as const;
 
+/**
+ * Halfway threshold on the 0→1 spring value. Above flips `isBack`,
+ * below flips `isFront`. Picking 0.5 means both states flip at the
+ * exact midpoint of the animation.
+ */
+const PERSPECTIVE_TOGGLE_THRESHOLD = 0.5;
+
 /**
  * Configuration options for perspective effects
  */
@@ -107,7 +114,7 @@ export class PerspectiveManager {
      * Content should appear blurred, scaled down, and less interactive
      * when this is true. Derived from spring value > 0.5.
      */
-    isBack = $derived(this.spring.current > 0.5);
+    isBack = $derived(this.spring.current > PERSPECTIVE_TOGGLE_THRESHOLD);
 
     /**
      * Reactive state: true when in front position
@@ -115,7 +122,7 @@ export class PerspectiveManager {
      * Content should be fully visible, sharp, and interactive
      * when this is true. Derived from spring value < 0.5.
      */
-    isFront = $derived(this.spring.current < 0.5);
+    isFront = $derived(this.spring.current < PERSPECTIVE_TOGGLE_THRESHOLD);
 
     /**
      * Internal configuration with defaults applied

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;
                         }
                     }