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).

core/dr-decode/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "dr-decode"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+
+[dependencies]
+dr-types.workspace = true
+rawler.workspace = true
+zune-jpeg.workspace = true
+thiserror.workspace = true
+log.workspace = true
+
+[dev-dependencies]
+env_logger.workspace = true