[S42yt]

This moves the installer out of burnlang/burn.

• install.sh installs a prebuilt release, or builds Burn from source with its scripts/package.sh. It falls back to source when the release is too old to build ash.
• It builds ash from its Burn source, using the interpreter when native compilation isn't available.
• It installs itself as burnup, with update, show, uninstall and help.
• It sets up PATH, and still recognises lines added by the old installer.
• uninstall deletes the whole prefix only when burnup created it. Otherwise it removes just its own files.

Tested locally: a from-source install from local checkouts, then burnup show, ash init + ash build + ash install, and burnup uninstall. shellcheck is clean.

CI builds Burn from master, so it goes green once burnlang/burn#14 and burnlang/ash#1 are merged.

🤖 Generated with Claude Code

https://claude.ai/code/session_01PCXUjgpGnka9y7AHKfnN3w

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Generated by Claude Code

[S42yt]

The install check fails with error: master is too old for burnup (it has no scripts/package.sh). It builds Burn from burnlang/burn master, and scripts/package.sh is added by burnlang/burn#14, so this goes green once that PR is merged. The step after it also needs to anonymously clone burnlang/ash, which needs burnlang/ash#1 merged and the ash repository made public. I'll re-run the check after those land.

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