Export: Targa (.tga) sequence output, 24-bit RGB or 32-bit RGB + Alpha

After Effects lists Targa among its still-image output formats; EffectCraft
imported Targa (#745) but could not render to it.

- OutputFormat::TargaSequence ("Targa Sequence", `.tga`, numbered like the
  other sequences; `tga` / `targa` as a format name or output extension).
- Written by the `image` crate's Targa encoder (its `tga` feature, already
  enabled in effectcraft-media; no new crates, Cargo.lock unchanged), with
  run-length compression off: an 18-byte header, top-left origin, 24-bit BGR
  for RGB, 32-bit BGRA with 8 alpha bits in the image descriptor for
  RGB + Alpha (straight or premultiplied), 8-bit grey for Alpha.
- The render queue's size estimate counts the uncompressed bytes.
- CLI usage, the renderQueue command specs, README, architecture.md and
  file-format-parity.md name the new format.

Tests: each Targa frame holds exactly the pixels of the PNG sequence of the
same job, with the expected header, numbering and size; the frames read back
through the Targa importer as the same picture (straight, and premultiplied
when interpreted so); the image-sequence round trip, the format parser and
the renderQueue commands cover it too.

crates/engine/src/render_queue.rs

@@ -208,6 +208,16 @@ pub fn bytes_per_frame(om: &OutputModule, w: u32, h: u32, fps: f64) -> f64 {
             let bpp = if om.channels == effectcraft_project::render_queue::Channels::Rgba { 4 } else { 3 };
             ((w as u64 * bpp).next_multiple_of(2) * h as u64) as f64
         }
+        // An 18-byte header and 24-bit BGR, 32-bit BGRA or 8-bit grey (Alpha) pixels, uncompressed.
+        OutputFormat::TargaSequence => {
+            use effectcraft_project::render_queue::Channels;
+            let bpp = match om.channels {
+                Channels::Rgb => 3,
+                Channels::Rgba => 4,
+                Channels::Alpha => 1,
+            };
+            (18 + w as u64 * h as u64 * bpp) as f64
+        }
         OutputFormat::WebM => mbps(8.0) * px / fps,
         OutputFormat::Gif => w as f64 * h as f64 * 0.5,
         OutputFormat::Wav | OutputFormat::Aiff => om.audio_sample_rate as f64 * 4.0 / fps,