Replace the C++ implementation with the Rust port

The Rust port (formerly gipfel-rs) is now the official successor to the
FLTK/GSL C++ code base. This squashes the port into a single commit and
removes the C++ sources (they remain in the git history below).

Layout:
  crates/gipfel-core  headless core: hills, panorama geometry, projections
                      (maxima-generated derivatives), pure-Rust LM fit,
                      gipfel: JPEG comment + EXIF I/O, profiles, stitching
  crates/gipfel-cli   command-line tools (export, info, stitch, profiles)
  crates/gipfel-app   Cosmic DE (libcosmic) GUI, binary name `gipfel`
  codegen/            maxima scripts + regen.sh for the projection code
  tests/              golden files + C++ reference harnesses (historical)

The original README is kept for now; it will be updated to describe the
Rust build. gipfel.dat, AUTHORS, ChangeLog and NEWS are unchanged.

1 files changed, 33 insertions, 0 deletions

diff --git a/README.md b/README.md
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+# gipfel-rs
+
+Rust port of [gipfel](https://git.flpsed.org/gipfel)'s computational core.
+
+- `crates/gipfel-core` — hills, panorama geometry, projections (maxima-generated
+  symbolic derivatives), pure-Rust Levenberg-Marquardt fit, `gipfel:` JPEG comment + EXIF I/O
+- `crates/gipfel-cli` — `export-position`, `export-hills`, `info`
+- `codegen/` — maxima scripts (`expr2rust.mac`) and `regen.sh`
+- `tests/` — C reference harnesses + golden files from the original C++ build
+
+```sh
+cargo test --lib --test golden     # unit + golden tests
+cargo run -p gipfel-cli -- info <img.jpg>
+```
+
+The Rust build has no native C dependencies. Regenerate goldens with
+`tests/gen_golden.sh` (needs the built C++ tree, which uses GSL; see
+`GIPFEL_SRC`/`GIPFEL_DATA` in the script).
+
+## gipfel-app (M2/M3)
+
+COSMIC desktop application: `cargo build --release -p gipfel-app` →
+`target/release/gipfel [image.jpg] [--data gipfel.dat]`.
+
+- Viewport: ctrl+wheel zoom (cursor-anchored), wheel/drag pan, fit/zoom buttons
+- Left-drag a flag to reposition (refits on release); middle-click or
+  double-click toggles a hill "known"; right-click → Find Peak / Hide-Unhide
+- Viewpoint / Find Peak pickers are searchable dialogs
+- Sidebar: compass dial, focal/pitch/roll/visibility/track sliders, viewpoint
+  lat/lon/height, distortion k0/k1/x0, projection, show-hidden
+- File menu: Open/Save (writes the `gipfel:` comment), Screen Dump (PNG/JPG
+  with markers burned in), Load Track (CSV: lat,lon,height rows)
+- Install `crates/gipfel-app/data/org.gipfel.App.desktop` per freedesktop