v0.2.9

The source-build fallback in scripts/fetch-or-build.sh ran rm -rf "$TMP",
and four fallbacks (no version, unknown platform, no curl, no sha256 tool)
fire before the script assigns TMP from mktemp. TMP is a conventional
environment variable, so an install in an environment that exports it
(TMP=/tmp, nix-shell, some CI images) had that directory deleted.

The script now keeps its temp directory in a private variable that starts
empty and removes only what it created; download, checksum and fallback
behaviour are unchanged. scripts/fetch-or-build-selftest.sh forces each of
the four early fallbacks against a sentinel directory, fails against the
previous script in all four cases, and runs in CI.

Bumps the version to 0.2.9.

Co-Authored-By: Claude Fable 5.1 noreply@anthropic.com

What's Changed

• fix(build): never delete an inherited TMP directory in fetch-or-build by @winoooops in #5

Full Changelog: v0.2.8...v0.2.9