Use a transparent lettering-only VDMT homepage wordmark

[Llewellynvdm]

Requested logo correction

Replace the boxed decorative homepage image with a single transparent PNG reading VAST DEVELOPMENT METHOD THEORY on one baseline. The original wording and T/H/E/O letterforms are extracted from the supplied VDM logo; matching R/Y forms complete THEORY. No claim is made that the original font file was recovered.

• Remove the green mosaic/decorative background and the separately typeset THEORY caption.
• Remove the white background plate and border from the homepage wordmark container.
• Let the page's actual light/dark background show through the PNG's alpha channel. A scoped dark-theme brightness adjustment preserves readability and follows the existing effective theme, including manual overrides.
• Keep the wordmark responsive; retain an accessible complete text alternative.
• Remove the obsolete WebP and add two tests for PNG transparency and single-image presentation.

Scope: seven files in one focused commit. No paper articles, compiler documentation, deployment workflows, favicon, header logo, or theme-switching logic are changed.

Validation complete

Passing publication workflow: https://github.com/vast-development-method/theory/actions/runs/34991828875

• 49 unit tests passed.
• Publication build and 62 HTML/Markdown pairs passed verification.
• 62 desktop and 62 mobile article renderings passed the existing browser checks; no browser errors were recorded.
• System appearance, persistent manual theme overrides, search, and mobile navigation passed.
• The new wordmark's real alpha channel and single-image/no-caption markup are covered by the new tests. Its transparent CSS surface was also checked in locally rendered light/dark previews.
• CI light/dark/mobile screenshots and reports are retained in the vdmt-publication artifact. The published PNG bytes were retrieved and compared with the local approved asset.

The remote Git tree matches the locally tested tree exactly: 974530a7922021818374714fb4ff915d13118f14. The PNG blob matches the local asset: 87508410acd0e0cf717cedc3eb08b88b1bdffad0 (15,803 bytes).

Ready for review and merge. Not merged automatically.