Per glyph, ShapeText did cache_.contains(key) (in Ensure) then
cache_.find(key) (in Lookup) — two independent hashes + node-chases of
the same key. Add EnsureAndGet returning const Glyph* with a single
find, insert-on-miss, returning &it->second. It returns nullptr only on
ShelfPlace failure, preserving the existing `if (g==nullptr) continue;`
behavior. Ensure now delegates to it.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
