[Feature] Expose Present Mode and VSync Configuration in RenderContextBuilder

[vmarcella]

Overview

Plumb the existing with_vsync flag from WindowBuilder through to RenderContextBuilder (Or one of it's settings) and expose explicit PresentMode selection for users.

Current State

WindowBuilder has a with_vsync method, but it is currently a no-op:

// crates/lambda-rs/src/render/window.rs
/// Request vertical sync behavior for the swapchain.
///
/// Note: present mode is ultimately selected when configuring the rendering
/// surface in `RenderContextBuilder`. This flag is reserved to influence
/// that choice and is currently a no‑op.
pub fn with_vsync(mut self, vsync: bool) -> Self {
  self.vsync = vsync;
  return self;
}

And within the platform layer, we already support present mode selection:

// crates/lambda-rs-platform/src/wgpu/surface.rs
pub enum PresentMode {
  Fifo,        // VSync on
  Immediate,   // VSync off, may tear
  Mailbox,     // Triple buffering
  AutoVsync,
  // ...
}

But RenderContextBuilder::build() currently hardcodes PresentMode::Fifo for the surface.

Scope

• Add with_vsync(bool) to RenderContextBuilderthat maps toFifowhen true, or Immediate/Mailbox` when not.
• Add with_present_mode(PresentMode) for explicit control
• Expose higher level PresentMode enum in lambda-rs
• Validate requested mode against adapter capabilities; fall back to something that is supported if the requested mode is not available.

Proposed API

// crates/lambda-rs/src/render/mod.rs
pub enum PresentMode {
  /// VSync enabled, capped to display refresh rate (FIFO).
  Vsync,
  /// VSync disabled, immediate presentation (may tear).
  Immediate,
  /// Triple buffering, low latency without tearing if supported.
  Mailbox,
}

impl RenderContextBuilder {
  /// Enable or disable vertical sync (defaults to enabled).
  pub fn with_vsync(mut self, enabled: bool) -> Self;
  
  /// Explicitly select a presentation mode.
  pub fn with_present_mode(mut self, mode: PresentMode) -> Self;
}

Acceptance Criteria

• RenderContextBuilder::with_vsync(bool) to enable vsync
• RenderContextBuilder::with_present_mode(PresentMode) for explicit selection
• PresentMode implementation in lambda-rs
• Adapter validation and fallback support
• Documentation and example Usage