feat(ads/seo/release): v1.2.0 with live Google Ads, isolated ad profile, ads.txt, and SEO/AEO/GEO optimization

src/Paca.Core/VideoSpriteSheetGenerator.cs

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+using System;
+using System.Text;
+
+namespace Paca.Core;
+
+/// <summary>
+/// Configuration for tiled thumbnail sprite sheet generation.
+/// </summary>
+public sealed record SpriteSheetConfig(
+    int Columns,
+    int Rows,
+    int ThumbWidth,
+    int ThumbHeight,
+    int IntervalSec)
+{
+    public int TotalTilesPerSheet => Columns * Rows;
+    public int SheetWidth => Columns * ThumbWidth;
+    public int SheetHeight => Rows * ThumbHeight;
+}
+
+/// <summary>
+/// Generates ffmpeg tile filter pipelines and WebVTT sprite coordinate mappings for scrubbing preview.
+/// </summary>
+public static class VideoSpriteSheetGenerator
+{
+    /// <summary>
+    /// Builds ffmpeg CLI command line for rendering tiled thumbnail sprite sheet JPEG.
+    /// </summary>
+    public static string BuildFfmpegSpriteCommandLine(
+        string inputVideoPath,
+        string outputJpgPath,
+        SpriteSheetConfig config)
+    {
+        if (config.IntervalSec <= 0) throw new ArgumentOutOfRangeException(nameof(config.IntervalSec), "Interval must be positive.");
+        if (config.Columns <= 0 || config.Rows <= 0) throw new ArgumentOutOfRangeException(nameof(config), "Columns and Rows must be positive.");
+        return $"-i \"{inputVideoPath}\" -vf \"fps=1/{config.IntervalSec},scale={config.ThumbWidth}:{config.ThumbHeight},tile={config.Columns}x{config.Rows}\" -frames:v 1 \"{outputJpgPath}\" -y";
+    }
+
+    /// <summary>
+    /// Generates standard WebVTT subtitle/scrubber specification with #xywh coordinates.
+    /// </summary>
+    public static string GenerateVttMapping(
+        string spriteFileName,
+        double totalDurationSec,
+        SpriteSheetConfig config)
+    {
+        if (config.IntervalSec <= 0) throw new ArgumentOutOfRangeException(nameof(config.IntervalSec), "Interval must be positive.");
+        if (config.Columns <= 0 || config.Rows <= 0) throw new ArgumentOutOfRangeException(nameof(config), "Columns and Rows must be positive.");
+
+        var sb = new StringBuilder();
+        sb.AppendLine("WEBVTT");
+        sb.AppendLine();
+
+        var tileIndex = 0;
+        for (double start = 0; start < totalDurationSec; start += config.IntervalSec)
+        {
+            var end = Math.Min(start + config.IntervalSec, totalDurationSec);
+            var startTimeStr = FormatVttTime(start);
+            var endTimeStr = FormatVttTime(end);
+
+            var col = tileIndex % config.Columns;
+            var row = (tileIndex / config.Columns) % config.Rows;
+            var x = col * config.ThumbWidth;
+            var y = row * config.ThumbHeight;
+
+            sb.AppendLine($"{startTimeStr} --> {endTimeStr}");
+            sb.AppendLine($"{spriteFileName}#xywh={x},{y},{config.ThumbWidth},{config.ThumbHeight}");
+            sb.AppendLine();
+
+            tileIndex++;
+        }
+
+        return sb.ToString();
+    }
+
+    private static string FormatVttTime(double seconds)
+    {
+        var ts = TimeSpan.FromSeconds(seconds);
+        return $"{(int)ts.TotalHours:D2}:{ts.Minutes:D2}:{ts.Seconds:D2}.{ts.Milliseconds:D3}";
+    }
+}