ci: run the workspace tests on every change; drop the unused flake

- .github/workflows/ci.yml runs `cargo test --workspace --locked` on
  pushes to main and on pull requests, with the Linux libraries the
  release build installs. No formatter check: the tree is not
  rustfmt-clean and stays that way.
- flake.nix, flake.lock and the direnv hook are removed. Nothing used
  them, and their pinned dependency hashes were already stale after the
  codec and kernel bumps.
- Under cfg(test) the "system" language resolves to en-US. The unit
  tests compare English text through one process-wide loader, and every
  apply_config of a default config asked for the system language, so a
  Turkish locale failed 22 tests and parallel tests raced each other
  through a language switch (the thickness-row test found its section
  under one language and looked it up under another).
- The arc grip test follows #1352: the start, end and midpoint grips
  drive the whole arc.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

.gitignore

@@ -36,9 +36,6 @@ __pycache__/
 # Claude Code local preview/dev-server config
 .claude/launch.json
 
-# nix-direnv
-.direnv/
-
 # Worktrees created for isolated runs
 .claude/worktrees/