Release prep 0.7.0: Linux riscv64 tarball + version bump (#2965)

* Release prep 0.7.0: add Linux riscv64 tar.gz to the release; bump to 0.7.0

- release.yml: new linux-riscv64 job, ported from GridCraft: cross-compiled on ubuntu-24.04
  against ports.ubuntu.com riscv64 libs, tar.gz only, photocraft-cli --version smoke-tested
  under QEMU. The draft-release job now needs it, so SHA256SUMS.txt covers it.
- packaging/linux/package.sh: CROSS_ARCH / CROSS_TARGET / CROSS_COMPILE / EMULATOR support.
- linux_libs: search /usr/lib/riscv64-linux-gnu on riscv64 (was the x86_64 triplet).
- README and docs/releasing.md list the riscv64 tarball.
- Version 0.6.0 -> 0.7.0 (cargo xtask version set), Cargo.lock refreshed; contributors refreshed.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

* canvas: crisp elliptical marquee ants again in an unrotated view

#2732 (a27d7175) built the elliptical marquee's live ants in document space and snapped every
vertex to the half-pixel grid on its own. Near the edge midpoints the nearly flat arcs became
1 px stairs, whose antialiased steps are grey, so the ants no longer contrast on white or black:
drag_preview_canvas's marquee test fails on main (ubuntu and windows). In an axis-aligned view
(rotation a multiple of 90 degrees) draw the ellipse inscribed in the snapped screen box again,
as before #2732; rotated views keep the mapped ellipse.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

---------

Co-authored-by: Claude Opus 5.5 <[email protected]>

Cargo.toml

@@ -4,7 +4,7 @@ members = ["crates/*", "apps/*", "xtask"]
 default-members = ["crates/*", "apps/photocraft", "apps/photocraft-cli", "xtask"]
 
 [workspace.package]
-version = "0.6.0"
+version = "0.7.0"
 edition = "2024"
 license = "MIT OR Apache-2.0"
 rust-version = "1.95"