chore: replace meval with expr-solver-lib, drop reqwest from runtimes, fix AUR deps

- owlry-core: swap meval → expr-solver-lib for calculator and Lua math API;
  add ln() alias for meval compatibility
- owlry-lua: remove reqwest and meval (network features belong in plugins);
  add vendored feature flag so distro builds can link against system lua54
- owlry-rune: remove reqwest (same reason)
- aur/owlry-rune: fix depends (gcc-libs only; owlry-core → optdepends)
- aur/owlry-lua: fix depends (gcc-libs + lua54; owlry-core → optdepends)
- aur/owlry: add chmod -R a+rX for example plugins
- justfile: log aur-local-test output to build-logs/
- .gitignore: exclude build-logs/ and test-build-output files
- README: minor improvements (SIGHUP reload hint, plugin_config example)

.gitignore

@@ -12,3 +12,8 @@ aur/*/*.tar.zst
 aur/*/*.tar.gz
 aur/*/*.tar.xz
 aur/*/*.pkg.tar.*
+
+# Local AUR test build logs
+build-logs/
+test-build-output*.md
+test-build-output*.log