v0.11.7

One new rule for a class nothing was checking: elements that repeat where the
spec allows one. A tag that validated clean on 0.11.6 can report findings
here. Catalog: 222 to 223.

Added

• VAST-2.0-duplicate-singular-element (error). Every VAST schema from 2.0 to
  4.2 caps children such as <AdSystem>, <AdTitle>, <AdServingId>,
  <VASTAdTagURI>, <Duration> and <ClickThrough> at one occurrence per
  parent. Nothing here had ever counted them, and a repeat is genuinely
  ambiguous: two <VASTAdTagURI> in a wrapper do not say which chain hop the
  player should follow, and two <Duration> in a <Linear> do not say how long
  the ad is.

  The gap was invisible while the XSD covered it. vast_4.4.xsd wraps
  vastInLine_type and vastWrapper_type in
  <xs:choice minOccurs="0" maxOccurs="unbounded">, which drops cardinality for
  every child it contains, so on a 4.4 document neither the schema nor this
  validator noticed. Reported upstream as IAB VAST issue #58 with a fix in PR
  #59; the rule stands regardless of how that lands, because the prose has
  required these cardinalities since 2.0.

  Forty (parent, child) pairs, derived from vast_4.2.xsd rather than by hand,
  dispatched through the element walker so a container is covered wherever it
  appears. Verified against the full IAB VAST_Samples corpus and every fixture
  in this repo: 196 files, zero false positives. A guard test fails the build if
  the rule ever fires on a fixture not named err_duplicate_*.

Changed

• Rule count corrected across the surfaces that had drifted: ROADMAP.md,
  docs/tutorial.md, docs/mcp-agentic.md, the vastlint-core crate
  description, the MCP server.json, and the Chrome, VS Code and npm package
  manifests all quoted 220 against a catalog of 222.

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