M13.2: Photo Merge commands, dialog with live preview, CLI

- engine: `merge.hdr`, `merge.panorama`, `merge.hdrPanorama` (bracket size from
  EXIF); MergeJob (Send) with progress/cancel, preview renders through the develop
  pipeline (deghost overlay, auto-crop frame); finish writes <name>-HDR/-Pano.dng
  next to the first source, imports, selects, applies Auto Settings / auto crop
- ui: Photo Merge dialog (HDR / Panorama / HDR Panorama) with live preview on a
  worker thread, background final merge with progress toast; context menu and
  Ctrl+H / Ctrl+M (Ctrl now means the physical Control key on macOS)
- cli: `merge hdr|panorama|hdr-panorama`, `synth-merge` for synthetic inputs
- merge::synth: procedural brackets (DNG) and panorama views; docs/merge.md

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/merge.rs

@@ -0,0 +1,72 @@
+//! Photo Merge commands: `merge.hdr`, `merge.panorama`, `merge.hdrPanorama`.
+//!
+//! Run synchronously here (CLI, MCP, control channel); the desktop UI plans the same job with
+//! [`Session::plan_merge`] and runs it on a worker thread. `preview: true` merges ≤ 1024 px frames
+//! and (with `previewPath`) writes the rendered preview as PNG instead of adding a photo.
+
+use lightcraft_catalog::PhotoId;
+use serde_json::{Value, json};
+
+use super::{CommandSpec, bad, bool_or, cmd, str_param};
+use crate::{Result, Session};
+
+fn can_merge(s: &Session) -> std::result::Result<(), String> {
+    if cfg!(target_arch = "wasm32") && s.media.file_bytes.is_none() {
+        return Err("Photo Merge needs access to the photo files".into());
+    }
+    // the photo count is checked when run (an explicit `ids` parameter may name them)
+    if s.catalog.photos().nth(1).is_none() { Err("the library has fewer than 2 photos".into()) } else { Ok(()) }
+}
+
+fn run(s: &mut Session, id: &str, p: &Value) -> Result<Value> {
+    let (kind, finish) = crate::merge::parse(id, p)?;
+    let ids: Vec<PhotoId> = s.targets(p);
+    if !bool_or(p, "preview", false) {
+        return s.merge_now(kind, finish, &ids);
+    }
+    let job = s.plan_merge(kind, finish, &ids, true)?;
+    let out = job.run(&|_, _| true).map_err(|e| bad(id, e))?;
+    let mut info = out.info;
+    if let (Some(path), Some(img)) = (str_param(p, "previewPath"), &out.preview) {
+        let png = lightcraft_codecs::encode_png(&lightcraft_codecs::EncodeImage::rgba8(img), &lightcraft_codecs::EncodeMeta::default())
+            .map_err(|e| bad(id, e.to_string()))?;
+        std::fs::write(path, png).map_err(|e| bad(id, format!("{path}: {e}")))?;
+        info["previewPath"] = json!(path);
+    }
+    if let Some(img) = &out.preview {
+        info["previewSize"] = json!([img.width, img.height]);
+    }
+    Ok(info)
+}
+
+pub fn specs() -> Vec<CommandSpec> {
+    vec![
+        cmd!(
+            "merge.hdr",
+            "HDR Merge",
+            [],
+            None,
+            "{ids?, align=true, deghost=none|low|medium|high, autoSettings=true, stack=false, preview=false, showOverlay=false, previewPath?}",
+            can_merge,
+            |s, p| run(s, "merge.hdr", p)
+        ),
+        cmd!(
+            "merge.panorama",
+            "Panorama Merge",
+            [],
+            None,
+            "{ids?, projection=auto|spherical|cylindrical|perspective, boundaryWarp=0..100, autoCrop=false, fillEdges=false, autoSettings=true, stack=false, maxMegapixels=40, preview=false, previewPath?}",
+            can_merge,
+            |s, p| run(s, "merge.panorama", p)
+        ),
+        cmd!(
+            "merge.hdrPanorama",
+            "HDR Panorama Merge",
+            [],
+            None,
+            "{ids?, bracket=0 (auto from EXIF), align, deghost, projection, boundaryWarp, autoCrop, fillEdges, autoSettings, stack, preview, previewPath?}",
+            can_merge,
+            |s, p| run(s, "merge.hdrPanorama", p)
+        ),
+    ]
+}

crates/engine/src/cmd/mod.rs

@@ -8,6 +8,7 @@ mod develop;
 mod edit;
 mod library;
 mod masks;
+mod merge;
 mod preset_files;
 mod query;
 mod xmp;
@@ -102,6 +103,7 @@ pub fn command_specs() -> &'static [CommandSpec] {
         v.extend(library::specs());
         v.extend(develop::specs());
         v.extend(masks::specs());
+        v.extend(merge::specs());
         v.extend(query::specs());
         v.extend(xmp::specs());
         v.extend(preset_files::specs());