Declare a bare sigil: minimum

The sigil: field carried a caret range ("^0.17"). It declares a MINIMUM, not a range -- compatibility resolves as >= declared -- and ranges have been refused since 2026-08-25, with the caret spelling warning on existing manifests. Brought to the bare 0.20 form used across the estate.

No other change was needed. This repo builds cleanly under its own manifest.scm, which already lists everything required including libglvnd for GL/gl.h. It was recorded as a build failure only because the verification sweep ran plain sigil build instead of guix shell -m manifest.scm, which is a defect in the instrument rather than in the repo.

Its released v0.11.1 likewise already carries the with-sigil-c-flags fix, so no release is needed for consumers either; sigil-studio was failing against it purely because it pinned ^0.7.0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01P6mPymR1ZBRV1nuyDpBxDF