Add folder scan with format selection; validate A3 on a real library

Library setup as the user described it: pick a folder, choose which RAW
types to look for, scan recursively.

  dr-types::FormatFilter  the tick-box selection, seeing through VFS
                          placeholder suffixes so a dehydrated CR2 still
                          matches as a CR2
  dr-sync::scan           recursive walk, Depth:1 per directory, pruning
                          unchanged subtrees where the backend propagates
                          directory ETags

Verified against nextcloud.tourolle.paris (34.0.2) on a real library:

  browse root      32 entries, 98ms
  scan PhotosRaw   17,185 RAW files in 334 directories, 34.1s
                   (7,836 CR2 + 9,349 DNG)
  range read       262KB of a 21.5MB DNG in 119ms — 1.22% of the file,
                   and enough to read "Canon EOS 6D | ISO 100"

That last line is assumption A3 validated on real data. Cataloguing this
library by whole-file fetch would move roughly 370GB; the range path
moves a few MB.

Pruning is capability-gated rather than assumed: with per-entry ETags a
probe costs a request and proves nothing about children, so it is skipped
entirely. A test asserts zero probes in that case.

Still unresolved: /core/preview returns 400 for every parameter
combination tried, including on a JPEG the server reports as having a
preview. Not a request-shape bug — it fails identically bare. Recorded
rather than worked around; ARCH §6.7 already treats server previews as
opportunistic, so nothing depends on it.

core/dr-pipeline/src/descriptor.rs

@@ -113,6 +113,37 @@ impl ParamDescriptor {
         }
     }
 
+    /// A toggle. Neutral when off, so the reset contract still holds.
+    pub const fn switch(id: &'static str, label: &'static str) -> Self {
+        Self {
+            id: ParamId(id),
+            label: LocalizedKey(label),
+            kind: ParamKind::Bool,
+            default: 0.0,
+        }
+    }
+
+    /// A 0…1 fraction — a proportion of something, rather than an amount.
+    ///
+    /// Its own constructor because the crop rect needs four of them and the
+    /// default differs per edge: an origin starts at 0 and an extent at 1.
+    /// Precision of 4 because at 6000px a step of 0.0001 is under a pixel,
+    /// and a coarser one would make a crop edge unplaceable.
+    pub const fn fraction(id: &'static str, label: &'static str, default: f32) -> Self {
+        Self {
+            id: ParamId(id),
+            label: LocalizedKey(label),
+            kind: ParamKind::Scalar {
+                min: 0.0,
+                max: 1.0,
+                scale: Scale::Linear,
+                unit: Unit::None,
+                precision: 4,
+            },
+            default,
+        }
+    }
+
     /// A general scalar with an explicit range and default.
     //
     // Eight arguments, and a builder would be the usual answer — but this has