HermitCrab feature structures: skip the clone map for frozen flat trees (no isolated measurement) #587

FeatureStruct.Clone builds an old-to-new copies map on every clone. A frozen structure marked as a flat tree has no aliases or cycles below its root, so a top-level clone cannot meet the same value twice and needs no map. The change skips the map in that case. Other clones borrow a per-thread map, clear it in finally, and use a fresh map when reentered. The internal recursive constructor always uses the caller's map. This is a per-thread pooled collection, not a cross-parse cache. The change is on the archived branch perf/hc-optimization-archive.

Measured. Nothing in isolation. It was timed only as part of the package, so the record makes no speed claim.

What a revival must do first. Measure it alone, in allocations or dictionary operations avoided per clone. Keep FeatureStructCloneTests (aliases, cycles, reentrancy, a tree embedded in a shared graph) with it.

Record: https://github.com/sillsdev/machine/blob/2eba2f502338705a4d529d7034cecb6303db7f35/docs/optimizations/feature-struct-frozen-tree-clone.md
Ledger: https://github.com/sillsdev/machine/blob/2eba2f502338705a4d529d7034cecb6303db7f35/docs/hermitcrab-optimization-ledger.md; disposition: https://github.com/sillsdev/machine/blob/2eba2f502338705a4d529d7034cecb6303db7f35/docs/hermitcrab-perf-2026-09-disposition.md
From #490 (closed).