Use single-selection mapper API for default Classic facade mapping #259

Problem

Tiny f8a0909 + parser faccb2c9 rejects comparison-heavy-x64 (11,456CP) with E-MAPPING-LIMIT: preferred candidates even though the same committed CST maps successfully with the existing single-selection public API and complete source span. complex-x64 additionally triggers a separate quadratic reachability guard upstream. Both inputs are established positive JMH FacadeState fixtures. Existing Classic benchmark manifest13 does not contain x64; those smaller successful observations are not changed.

P4SourceMapping.mapOnce eagerly asks generated mapParsedTree/mapSubtreeTree for every candidate, even when the caller requested only default selection. This needlessly applies the bounded full-inventory4096cap to a one-result production use. Related parser#197/#184 and Tiny#256; no grammar/root-choice/retry changes belong in this issue.

Acceptance criteria

• Add a default-only adapter using existing selectParsedTokenWithSourceMap/selectSubtreeTokenWithSourceMap (legacy capability fallback unchanged), mapping exactly once and retaining the selected AST/owned source for repeated default selection.
• Use one retained default AST when allowDefault=true; preserve the facade's existing implicit candidate order, whole-source filtering, fallback, AST and selectionMode by selecting whole-source wrapper nodes already present in that AST. A default request has nonempty implicit candidates, so candidates.isEmpty() is not a valid discriminator. Explicit result preferences continue using the existing full-inventory path. Explicit preferred snapshot remains on existing API/candidate4096/work limits; no fallback retry on projection failure, no reparse, no cap increase.
• Compare the old full-inventory selection and single-AST implicit selection on the same committed CST across the existing corpus (including every eligible whole-source implicit preference), with independent small expected AST/mode and large x64 outcomes. Test root/alternate/legacy capability paths, same AST instance/snapshot on repeated default selection, source CP/full-span, exceptions propagated, no inventory API call, no preferred-cap bypass.
• Reproduce established x64 default input acceptance on fixed Java21 artifact without claiming all explicit-preferred large inventories are supported. The independent upstream guard fix handles complex-x64. Keep Java/Rust actual production facade semantics/CP oracle and existing shared corpus tests, separate from Classic SAFE performance. Coordinate source changes with concurrent Tiny256 facade projection.
• Validate relevant P4 source-mapping/diagnostics and Java/Rust shared acceptance tests, preserve backend pins/production grammar and memo defaults. Archive hashes/source revision/raw outcomes; no dedicated-host latency claim.

One reversible adapter-only unit; PR closes this issue, not parser197/184 or Tiny256.

Integration boundary

Tiny #256 changes canonical root projection and removes parse retries independently. Keep its projection when composing this adapter; do not treat default AST alone as the final facade result. Java Classic uses this Java adapter; the Java UBNFC and Rust facade APIs are unchanged and remain shared-corpus comparison oracles. Public all-candidate inventories remain bounded, and no dedicated-host performance claim follows from these shared-host semantic checks.