Phase D — Sentinel enum + single-table registry

The single-table registry collapsed four per-kind sentinel position tables into one position-keyed EytzingerMap dispatched through a NodeRef enum. Before the refactor the registry held independent inline / block_leaf / block_open / block_close EytzingerMaps and Registry::node_at(pos) swept them in declaration order with four if let Some(...) = table.get(&pos) chains; the current shape is one binary search per lookup, with the variant tag carried on the entry itself.

Structural changes

old  : Registry { inline, block_leaf, block_open, block_close }   // 4× EytzingerMap
       node_at(pos) → 4-way if-let chain, ~4 binary searches worst-case

now  : Registry { table: EytzingerMap<u32, NodeRef<'src>> }       // 1× EytzingerMap
       node_at(pos) → one binary search, NodeRef variant tags the kind

Renderers (crates/aozora-render/src/html.rs, crates/aozora-render/src/serialize.rs) replaced the parallel 4-way if let Some(...) = registry.<kind>.get(...) chains with a single (Structural, NodeRef) cross-product match — the compiler now enforces variant coverage at the call site.

Expected runtime impact

Theoretical: per-lookup binary search count drops from ≤ 4 to 1. Render hot path is dominated by registry lookups inside the memchr2_iter loop in html::render_into (one lookup per PUA sentinel hit), so the savings scale with sentinel density. Aozora corpus profiling against the four-table layout showed registry lookups at ~12 % of render time on bouten-heavy documents; the unified dispatch should absorb roughly that fraction.

Measuring before / after

The repro recipe lives in perf/samply.md. Numerical comparisons against the previous release are produced as release-PR artefacts (the corpus-sweep run output in /tmp/aozora-corpus-<timestamp>.json.gz, plus the diff produced by xtask trace compare) and summarised in the CHANGELOG entry for the release that lands the change. Pinned numbers in this page would rot; the recipe + per-release artefact pair stays current without an editing step here.