fix: require viewport-base.css in Mode C

[snvtac]

Summary

• Add a Mode C preflight that reads viewport-base.css before modifying an existing HTML deck.
• Require Mode C to verify the fixed-stage CSS signature and insert the full base CSS when it is missing or incomplete.
• Update the plugin-packaged SKILL.md copy and supporting-files table so root and plugin guidance stay in sync.

Root Cause

Mode C enhancement bypassed Phase 3, where the skill already required copying the full viewport-base.css contents into generated presentations. Existing HTML decks without that CSS could pass the Mode C viewport checklist while missing the .deck-stage and .slide.active / .slide.visible rules needed for fixed-stage rendering.

Validation

• git diff --check
• Confirmed root SKILL.md and plugin SKILL.md remain byte-identical
• Preflighted open issue/comments/PRs/remote branches and found no existing upstream fix

Fixes #87