feat(scripts): add an HTML tag-balance validator and a speaker-note confidentiality step

[Reebz]

What

Two authoring gaps that surfaced shipping a real deck.

• HTML tag-balance validator. scripts/validate-html.py (added) is a stdlib-only html.parser.HTMLParser subclass that tracks a stack of opened non-void tags and exits non-zero, naming the offending tag, on any unclosed or mismatched tag. No install step, matching the existing scripts/extract-pptx.py style. It is wired into the Step 5.5 validate-and-save flow and the after-any-modification block, listed as a ninth check in references/presentation-layer.md, and added to the supporting-files table. A test beside the script covers unclosed, mismatched, well-formed, and void-element cases.
• Speaker-note confidentiality. A public deploy ships the whole file, including every inline <script class="slide-notes">, so notes are readable by anyone with the URL. Phase 7A now grep-checks the file for client and project identifiers across visible text and the notes blocks, and when notes must not ship it deploys a stripped copy with every notes block removed while the original keeps notes for the presenter app. references/presentation-layer.md gains a one-line warning that notes ship in the public artifact unless stripped.

Test

• An unclosed div exits non-zero and names the tag.
• A well-formed deck exits zero.
• A mismatched closing tag is reported rather than silently passed.
• Void elements such as img, br, and input do not count as unclosed.

Dependencies

Independent of the runtime and CSS PRs, so it can merge at any time.

Co-Authored-By: Leslie Barbara Knope (Claude Opus 4.8 (1M context)) noreply@anthropic.com

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