[IsuraManchanayake]

From 0.4.3, the basert launcher places a release itself with basert self install --from <dir> --root <dir>. That is the layout basert self update keeps up to date:

• releases side by side under ~/.basert/versions/<v>/
• versions/current naming the one in use
• ~/.basert holding a link per release file through it, so the PATH line stays the same.

install.sh now unpacks the bundle into ~/.basert/versions/.staging/ and hands it to whichever launcher can place it:

1. The bundle's own basert (0.4.3 and later).
2. Otherwise the installed basert, when a pinned older release (BASERT_VERSION) goes over a 0.4.3+ install.
3. Otherwise nothing: it is unpacked flat into ~/.basert exactly as before, leaving the same files.

A copy an earlier run placed flat is kept as versions/<old>, the version basert self use <old> goes back to. The "already on " check is unchanged: .release is still written in ~/.basert, and ~/.basert/basert resolves through its link. The closing message mentions basert self update when the release can do it.

Safe to merge before 0.4.3 is published: a release without self install takes path 3.

Tested on macOS/arm64 with a stub curl serving local bundles:

• a fresh install of a release with self install
• an older release, fresh (flat, layout identical to today's script)
• that flat install, then the new release over it (the flat copy kept under versions/)
• a pinned older release over the layout (placed by the installed launcher)
• a re-run on the same tag (skipped).