Closes #3810

Summary

• make direct nvm-exec fall back to the default alias when NODE_VERSION and .nvmrc are absent
• preserve existing .nvmrc handling when a project file is present
• add fast test coverage with a fake installed default node version

Tests

• bash -n nvm-exec
• bash -n "test/fast/Running 'nvm-exec' should display required node version"\n- ./setup && "./Running 'nvm-exec' should display required node version"; code=$?; ./teardown; exit $code from test/fast\n- git diff --check\n- make TEST_SUITE=fast test-bash URCHIN=./node_modules/.bin/urchin was also attempted; the updated nvm-exec test passed, while the broader run failed on existing local-environment-sensitive cases involving color expectations, sudo, missing Linux loader/ldd, /dev/tty, and platform fixture output.