Add {vector-sizing-calculator} directive

[KOTungseth]

Summary

Wire the {vector-sizing-calculator} custom directive into elastic/docs-builder so the
interactive vector sizing calculator can be embedded in docs-content pages.

This is a prerequisite for the docs-content embed PR and must merge first.

Background

An open PR in elastic/docs-builder stages the React + EUI web component, directive examples,
and integration docs under tools/vector_sizing_calculator/. That PR includes three versions of
the calculator. Before opening this directive PR, Docs Eng must align on which version
to build from.

Scope

• Add directive files under src/Elastic.Markdown/Myst/Directives/VectorSizing/:
  • VectorSizingBlock.cs
  • VectorSizingView.cshtml
  • VectorSizingViewModel.cs
• Apply patches from DirectiveBlockParser.patch.md:
  • DirectiveBlockParser.cs — add VectorSizingBlock case before the UnknownDirectiveBlock fallback
  • DirectiveHtmlRenderer.cs — add WriteVectorSizing method and VectorSizingBlock case in the Write() switch
• Update base HTML template — add <script src="/_static/vector-sizing-calculator.iife.js" defer></script>
• Ship the compiled web component bundle (vector-sizing-calculator.iife.js) via standard bundle delivery

Out of scope

Formula updates, UI enhancements, and new element type support are owned by the Vector Search
team and tracked separately.

Prerequisites

• Version selection decision resolved on the open docs-builder PR
• BBQ constant accuracy question resolved (see formula accuracy issue)

Acceptance criteria

• {vector-sizing-calculator} directive is recognized by the build system without errors
• The compiled JS bundle loads correctly on pages where the directive is used
• The directive renders a <vector-sizing-calculator> custom element in the output HTML
• Integration guide verification checklist passes (element types, quantization combinations,
  HNSW and DiskBBQ advanced settings, cluster totals, formula panel accuracy)

Owner

Docs Eng

[itsalexcm]

Update from Docs Eng / contributor work on feat/vector-sizing-calculator:

Done (merged to branch, ready for PR review):

• Directive implementation under src/Elastic.Markdown/Myst/Directives/VectorSizing/ (VectorSizingBlock, VectorSizingViewModel, VectorSizingView).
• Registration in DirectiveBlockParser and rendering in DirectiveHtmlRenderer (Razor slice outputs <vector-sizing-calculator></vector-sizing-calculator>).
• Unit tests: VectorSizingBlockTests (directive parses and HTML contains the custom element).
• Staging docs under tools/vector_sizing_calculator/ updated: duplicate C# snippets removed from docs-builder-directive/; integration notes point at the in-repo source of truth.

Preview for reviewers:

• Docset page with a live directive block: docs/syntax/vector-sizing-calculator.md (also linked from docs/syntax/directives.md and _docset.yml).
  Local check: docs-builder serve → /syntax/vector-sizing-calculator.

Note vs issue checklist: the interactive widget is loaded via the existing site asset pipeline (main.ts dynamic import → VectorSizingCalculatorComponent), not a standalone <script src="/_static/vector-sizing-calculator.iife.js">. If the acceptance bar is strictly the _static IIFE, we should either add that artifact or align the issue wording with the agreed delivery path.