Add a test suite (sourceable script + stubbed side-effects)

[VijitSingh97]

Goal

RigForge has no tests (only shellcheck). Bring it to pithead's bar (which sources its script and black-box tests the CLI with docker/sudo stubbed).

Proposed

• Make rigforge.sh sourceable without side effects — guard main "$@" behind a BASH_SOURCE/_SOURCED check (as pithead does) so functions can be tested in isolation.
• Black-box test the pure logic with sudo/apt-get/git/make/systemctl/modprobe stubbed on PATH: parse_config, the .local short-hostname handling (:119), DONATION validation, the jq config generation (assert the produced XMRig config), and CPU-profile selection (EPYC / X3D / macOS branches).
• Live under tests/.

Acceptance

• rigforge.sh is sourceable; a test runner exercises the logic above with no real side effects.
• Config-generation output asserted for representative CPUs/OSes.