Fix slide Markdown lint without changing rendering

[codebytes]

Changes

• Correct 18 H3 subsections to H2 on ten slides, retaining their original size, color, margins and padding with existing scoped-style conventions.
• Remove the single MD009 trailing space from the slide 33 HTML column wrapper.
• Change only slides/Slides.md (+35/-27). No lint suppression, theme, publishing workflow, README, artwork, identity, example or presenter-note changes.

Verification

• Reproduced the current-main failure at e5af0d970461616ed2ae18da3bc822fe72736acb: ten MD001 errors and one MD009. The configured Super-linter v8.7.0 Markdown command (markdownlint-cli 0.49.0 / markdownlint 0.41.0 with .github/linters/.markdown-lint.yml) now exits 0.
• All 42 slides, 27 fenced examples, seven comment/directive blocks, Marp presenter-note arrays, links and all other non-style content are preserved. Existing CRLF is retained; the whitespace check passes with command-scoped core.whitespace=cr-at-eol.
• Rebuilt with the actual publisher's Marp CLI 4.5.1 / core 4.4.0, exact HTML/PDF options and LANG=POSIX LC_ALL=POSIX. Rebuilt baseline HTML equals the published reviewed deck after generated scope-ID alpha-renaming.
• Actual browser SCREEN, 1920x1080: all 42 slides have identical geometry/computed styles, no loaded text overflow and no asset/network failures. Slides 10, 11, 15, 16, 18, 19, 20, 21, 24, 26 and 33 have zero changed screenshot pixels versus baseline; their corresponding PDF pages are also pixel-identical at 1920x1080. All 42 PDF pages retain text and dimensions.

This is a narrow follow-up to #177. The coordinator owns normal CI, guarded merge of tested head 3b7fe42bc8d8069700069746835bcaee2f9ab61e, and live publication verification.