Mode C skips required `viewport-base.css` inclusion

The Core Principles section of SKILL.md states (line 52):

When generating, read viewport-base.css and include its full contents in every presentation.

Mode A (New Presentation) and Mode B (PPT Conversion) both route through Phase 3, whose generation rules explicitly require:

Include the FULL contents of viewport-base.css in the <style> block

Mode C (Enhancement) routes through ### Mode C: Modification Rules instead, and never enters Phase 3. The Mode C verification checklist (step 4) verifies that the slide stage stays 16:9, that text doesn't overflow, that panels don't overlap, and that screenshots look correct — but it does not verify that viewport-base.css contents are present.

Consequence: if a user runs the skill in Mode C against an input HTML that doesn't already contain viewport-base.css, the output passes every Mode C check but violates the "every presentation" claim. The slide-visibility classes (.active, .visible) and the visibility / opacity / pointer-events rules from viewport-base.css (relied upon at line 47) are not in the output, so the fixed-stage rendering breaks.

Suggested fix

Add a Mode C step before any modification that:

1. Reads the existing HTML.
2. Checks for the viewport-base.css signature (e.g., a .deck-stage class definition).
3. If absent, splices the full contents of viewport-base.css into the <style> block before applying any other modification.

Equivalently, extend the Mode C checklist (step 4) with a fifth bullet: viewport-base.css contents present in <style> block.