Add struct-based SimVar set functionality

[bstudtma]

Summary

Introduce a new API to set multiple SimVars in one call using a strongly-typed struct:

• SimVarManager.SetAsync<T>(T values, CancellationToken ct = default)

Usage Example

// Get current position (includes altitude)
var original = await client.SimVars.GetAsync<Position>(cancellationToken);

// Log current altitude
Console.WriteLine($"Original Altitude: {original.Altitude:F2} ft");

// Create a new target position
var target = original;
target.Altitude = original.Altitude + 15.0;

// Set new altitude (and any other changed fields) via struct
await client.SimVars.SetAsync(target, cancellationToken);

Where Position is:

public struct Position
{
    /// <summary>Latitude in degrees.</summary>
    [SimConnect("PLANE LATITUDE", "degrees")]
    public double Latitude;

    /// <summary>Longitude in degrees.</summary>
    [SimConnect("PLANE LONGITUDE", "degrees")]
    public double Longitude;

    /// <summary>Altitude in feet (MSL).</summary>
    [SimConnect("PLANE ALTITUDE", "feet")]
    public double Altitude;
}

Tests

• Add unit tests to verify:
  • Mapping from annotated struct fields to SimVar definitions.
  • Successful multi-SimVar set with SetAsync<T>.
  • Round-trip restore of the original values after the test completes.
• Update existing tests to cover mixed types and no-op updates (unchanged fields).

Performance & Reliability

• Reduces multiple round-trips to a single call for grouped updates.
• Minimizes inconsistent intermediate states across related SimVars.

Author Information

Discord Username: bstudtma

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Checklist:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

[Copilot]

Pull Request Overview

Adds a struct-based API to set multiple SimVars in a single call, mirroring the existing struct-based GetAsync path.

• Introduces SimVarManager.SetAsync(T value, ...) to write annotated struct fields in one request
• Implements internal writer pipeline (IFieldWriter, SimVarFieldWriter, SimVarFieldWriterFactory) to pack struct data into the unmanaged buffer using the same field layout as the get path
• Adds a test validating altitude change via struct set and restore

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/SimConnect.NET/SimVar/SimVarManager.cs	Adds public SetAsync and internal SetStructAsync to build and send packed struct data to SimConnect.
src/SimConnect.NET/SimVar/Internal/SimVarFieldWriterFactory.cs	New factory to reflect annotated struct fields, infer types, and build writers and layout sizes.
src/SimConnect.NET/SimVar/Internal/SimVarFieldWriter.cs	Writer that marshals each field into the unmanaged buffer based on SimConnectDataType.
src/SimConnect.NET/SimVar/Internal/IFieldWriter.cs	Interface for writing a single field into a packed buffer.
tests/SimConnect.NET.Tests.Net8/Tests/SimVarTests.cs	Adds a test to validate struct-based SetAsync for Position (altitude + restore).

[AussieScorcher]

Amazing work! Will merge and be pushing a new SimConenct.NET version shortly :)