Update and verify curl installer for Scriptella 1.5

[ejboy]

Scope

Update and verify the existing one-command curl installer for the Scriptella 1.5 release. The installer model remains unchanged: download the published ZIP, verify its pinned SHA-256, unpack it under ${HOME}/.local/scriptella, and expose its bin directory on PATH.

Checklist

• Update the release version and GitHub tag.
• Update the archive URL and filename.
• Record and pin the SHA-256 for the actual published 1.5 ZIP.
• Verify the installer against the published 1.5 archive.
• Update public installation documentation if necessary.
• Run a clean disposable-home install smoke test.

Non-goals

Do not redesign the installer, add launcher aliases or wrappers, or revive the experimental installer-1.5 implementation. If 1.5 includes a new packaged launcher, document it after verifying the archive; no special installer machinery is required.