Report img without alt and interactive content in a; document parser gaps

[casoon]

Four constructs that vnu flags produced no finding when placed alone in an otherwise valid document. What each one turned out to be:

Case	Root cause	This PR
<img> without alt	Missing rule: only the figure-without-figcaption case existed	Fixed: assertion.elements.img-missing-alt, following vnu's general missing-alt branch and its exemptions (aria-label, aria-labelledby, non-empty title; role/other aria-* keep their existing, more specific rules)
<button> inside <a>	Missing rule: vnu's interactive-descendant check for a had no counterpart (only a few button-ancestor rules existed)	Fixed: elements-interactive-in-a pattern, vnu's full list (button, select, textarea, label, details, dialog, embed, iframe, a, non-hidden input, video/audio[controls], img/object[usemap], [tabindex], interactive roles), one finding per element in vnu's else-if order
stray </div>	Parse error not recorded by html5-parser 0.3.0: StrayEndTag is only emitted on the "any other end tag" path (</span>, </b>), not in the block, li, dd/dt or heading end-tag branches	Documented gap
misnested <b><i>…</b></i>	Parse error not recorded by html5-parser 0.3.0: the adoption agency algorithm repairs the tree without recording an error	Documented gap

The two parser cases can't be fixed in this crate. The dropped tokens leave no trace in the tree, the same reason the stray-<!DOCTYPE> gap is already documented in src/parse.rs. The fix is in html5-parser: record the parse error at the remaining §13.2.6.4.7 "in body" end-tag branches and adoption agency steps, release it, then bump the =0.3.0 pin here. Two #[ignore]d regression tests in src/lib.rs reproduce both cases.

Docs

• README: the parser layer and the "tree-construction-error tracking landed" note now say only part of the tree-construction errors are covered. There is a new "What's not covered" section.
• docs/guides/vnu-comparison.md and docs/guides/rule-ids.md describe the parser gap. The differential test does not show it: the corpus fixtures vnu flags for a stray </header> or </td> still count as true positives, through unrelated heading and table findings.
• Showcase: content-model.html now includes both new findings. The JSON was regenerated with cargo run --example findings, and the description in site/src/showcase.ts was updated.
• CHANGELOG Unreleased entry.

Validation

• cargo fmt --check, cargo clippy --all-targets -- -D warnings, cargo test (417 passed, 2 known-gap tests ignored)
• cargo test --release --test differential -- --ignored: true_positive=3745 true_negative=909 false_positive=0 false_negative=1 not_comparable=0. This is unchanged, because the corpus has no fixtures for these two new rules in either direction. No baseline update.
• Site not rebuilt locally; only a description string changed.

The msrv job was already failing on master before this change: locked dependencies need a newer Rust than the declared 1.85. Not addressed here.