Add RAW decode and a working image viewer

dr-decode exposes four entry points rather than one decode, because
callers differ sharply in what they need (ARCH §3.2): culling wants a
preview, the grid wants metadata, only develop and export touch sensor
data. Fusing them forces a full decode where a header read suffices,
which is why Lightroom stalls ~2s per image while culling.

Smoke-tested against 1,852 real Canon CR2 files (EOS 6D, ~27MB each):

  metadata      0.2ms   from a 256KB header read, no full decode
  preview     ~250ms   5472x3648, downscaled to 2048 for display
  jpeg          3.0ms

Two findings worth recording:

rawler 0.7.2's CR2 decoder implements only full_image; thumbnail_image
and preview_image are unimplemented trait defaults returning None. So
every rung of the preview ladder resolves to a full-resolution decode at
~250ms — 5x over NFR-P13's 50ms budget. CR2 does carry smaller IFDs, so
the fix is our own IFD walk or an upstream contribution. The ladder is
written now so that fixing it is a decoder change, not a change to every
caller. Recorded in milestone-v0.1 risks.

Preview.downscale_to bounds memory: a 5472x3648 RGBA preview is 79.8MB,
which exhausts a phone's budget after a handful of images. Box-filtered
so downscaled thumbnails do not alias.

Also fixed a RefCell double-borrow that panicked on first navigation —
`*x.borrow_mut() = *x.borrow() + 1` holds both borrows at once. Verified
with 10,000 programmatic navigations.

58 tests passing. Traceability 20.3% (29/143).

Cargo.toml

@@ -2,6 +2,7 @@
 resolver = "2"
 members = [
     "core/dr-types",
+    "core/dr-decode",
     "core/dr-gpu",
     "core/dr-sync",
     "ui/dr-ui",
@@ -19,6 +20,7 @@ repository = "https://github.com/dtourolle/DarkRoom"
 [workspace.dependencies]
 # Internal
 dr-types = { path = "core/dr-types" }
+dr-decode = { path = "core/dr-decode" }
 dr-gpu = { path = "core/dr-gpu" }
 dr-sync = { path = "core/dr-sync" }
 dr-ui = { path = "ui/dr-ui" }
@@ -45,6 +47,11 @@ async-trait = "0.1"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 base64 = "0.23"
+
+# Decode. rawler is the pure-Rust decoder (D2); zune-jpeg decodes the
+# embedded previews rawler extracts.
+rawler = "0.7"
+zune-jpeg = "0.4.21"
 bytemuck = { version = "1", features = ["derive"] }
 
 [profile.dev]