fix: normalize Lynx text metrics

[xuefei1313]

What changed

• preserve native Lynx measureText() ascent/descent metrics instead of discarding them
• normalize device-scaled font bounding metrics by the canvas DPR
• add a focused regression test
• add Lynx Tooltip and baseline-matrix smoke scenes for visual verification

Root cause

The Lynx context previously returned only text width and forced VRender to estimate ascent/descent with fixed font-size ratios. That made text layout diverge from browser Canvas while symbols retained the expected coordinates.

Impact

Lynx text layout can use native font metrics, keeping Tooltip text aligned without VChart- or Tooltip-specific offsets. Unsupported metrics still fall through to VRender's existing fallback behavior.

Validation

• Lynx demo visually verified on device
• rushx test --runInBand __tests__/unit/lynx-context.test.ts
• rushx compile in packages/vrender-kits
• Lynx demo lint and typecheck
• repository pre-push package test suite