refactor(cmd): back generic execution with shellout (#20)

[GG-O-BP]

Summary

Resolves #20 by backing generic command execution with the maintained shellout
package instead of the bespoke cmd_ffi shell wrapper, reducing custom FFI
surface while preserving the public API and cross-platform behavior.

Changes

• Add shellout ~> 1.8.0 dependency; route cmd.exec through shellout's
  synchronous command.
• Preserve the existing shell-command-string semantics:
  • Unix: /bin/sh -c
  • Windows: ComSpec /d /s /c
• Keep the public contract stable:
  • cmd.exec(command:) -> Result(Nil, CommandError)
  • failures surface as CommandFailed("execute command", reason)
• Introduce src/glendix/command.gleam (+ FFI) as the focused execution module,
  with a contract test that verifies the shellout.LetBeStdout SIGINT listener
  is cleaned up after a command completes.
• Retain the small custom FFI only where shellout/plinth do not fit
  (Gleam warning filtering), documented in the READMEs (en/ko/ja).

Design notes

• plinth/node/child_process was not adopted: it lacks synchronous completion,
  typed exit status, and typed stdio results needed here.
• Bridge, watcher, experimental-native, Node/npm shims, and Rollup/WASM code are
  intentionally left as custom.

Verification

• gleam format --check, gleam check, gleam build --warnings-as-errors,
  gleam docs build — pass
• gleam test --runtime bun — 142 passed, no failures
• Mechanical Gleam style + FFI export contract checks — pass
• Repository boundary / sibling dependency direction — pass
• ./scripts/verify.sh final (family-wide) — pass, including:
  • all standalone consumers (glendam/glendix/mendraw/mxpak)
  • all package tests
  • Glendam browser smoke/tests
  • widget .mpk build (com.glendix.family.GlendixLustreWidget)
  • Mendix full E2E — BUILD SUCCEEDED + post-boot browser smoke check
• Dependency source contract preserved: glendix -> mendraw remains Hex;
  pushed-commit dependency check passes.

Closes #20