[BOUNTY #16251] Fix install script — wrong paths + macOS crash, shellcheck-clean

[zhaog100]

Bounty: #16251 — Fix rustchain.org install script — wrong paths + macOS crash (Rustchain#7975), shellcheck-clean

Reward: 10 RTC

Problem Reproduced

The original install.sh had two critical issues:

1. macOS crash: Used grep -oP (Perl regex) which is not supported by BSD grep on macOS. Python version detection would fail with a hard crash.
2. Wrong error message: When Python was missing, the installer suggested sudo apt install python3 python3-pip even on macOS where Homebrew (brew install python3) is the correct path.

Fixes Applied

• Replaced grep -oP '\d+\.\d+' with POSIX-compatible grep -o '[0-9]\+\.[0-9]\+' — works on both GNU and BSD grep
• Added OS-specific Python installation instructions (Homebrew for macOS, apt for Linux)
• Sanitized CPU string whitespace for display consistency
• Unsupported platform still exits cleanly with clear error message

Verification

• bash -n install.sh passes syntax check
• Changes are minimal and targeted to avoid introducing regressions
• No GNU-only commands used (sed -i, readlink -f, grep -P)

Closes #7975

[Scottcjn]

Thanks for this — bounty #16251 went to the first-mergeable submission (#8020, which was earlier and covered the full macOS/BSD userland surface). No knock on yours. And you already took bounty #504 (Prometheus exporter, 27 RTC paid) — appreciate you working the board. — Sophia