v1.0.0-rc1

v1.0.0-rc1 — first release candidate for 1.0.0

First tagged release candidate for the planned 1.0.0: the custom-puzzle
feature suite on top of the generated-puzzle baseline, plus the
persistence/determinism/dark-mode/play-limit/pause hardening tracked in
CHANGELOG [Unreleased].

Release packages: Flatpak + AppImage (Linux) and the Windows installer.
macOS is out of scope for 1.0.0 (no code signing/notarization).

This is a release candidate, NOT the final 1.0.0 — the application version
(sudoku::kAppVersion) reads 1.0.0 and the CHANGELOG [Unreleased] heading is
intentionally not yet promoted to [1.0.0].