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/features/AdjustTypography/model/store/typographySettingsStore/typographySettingsStore.svelte.ts

@@ -28,6 +28,14 @@ import {
 } from '$shared/lib';
 import { SvelteMap } from 'svelte/reactivity';
 
+/**
+ * Epsilon for detecting "significant" base-size changes when reconciling
+ * the multiplier-derived display value back to the underlying baseSize.
+ * Differences below this threshold are treated as rounding jitter and
+ * skipped to avoid spurious storage writes.
+ */
+const BASE_SIZE_EPSILON = 0.01;
+
 type ControlOnlyFields<T extends string = string> = Omit<ControlModel<T>, keyof ControlDataModel>;
 
 /**
@@ -139,7 +147,7 @@ export class TypographySettingsStore {
                 const calculatedBase = currentDisplayValue / this.#multiplier;
 
                 // Only update if the difference is significant (prevents rounding jitter)
-                if (Math.abs(this.#baseSize - calculatedBase) > 0.01) {
+                if (Math.abs(this.#baseSize - calculatedBase) > BASE_SIZE_EPSILON) {
                     this.#baseSize = calculatedBase;
                 }
             });