node-pty's published npm tarball ships the POSIX `spawn-helper` with mode
0644 (no exec bit). node-pty `posix_spawnp`s that helper on macOS/Linux, so a
non-executable copy fails every embedded-terminal spawn with
`Error: posix_spawnp failed.`. Packaged builds are unaffected because
stage-native-deps.mjs chmods the staged copy, but the dev flow
(`npm run dev` -> `electron .`) resolves node-pty straight from node_modules,
which nothing chmods -- so the first terminal in dev always dies.
Restore the exec bit once, lazily, right before the first spawn, via a small
DI-testable helper. Idempotent: already-executable copies (packaged builds)
are left untouched, and stat/chmod failures are collected and logged rather
than thrown so terminal startup never breaks.
