HermitCrab engine: share frozen Shapes on internal Word clones (clone 36% to 9% of profile; peak memory 11 GB to 6 GB) #593

Internal Word clones deep-copy the whole Shape (every node, feature structure and annotation), but three of the four clone sites never use the copy. The change adds Word.CloneForEngine, which shares a frozen source shape and marks it shared. EnsureOwnShape copies on demand before in-place mutation, and is called in AnalysisStratumRule and SynthesisStratumRule. ResetShape and Shape.CreateEmptyLike replace clone-then-clear at the four GenerateShape sites. Public Word.Clone() still returns a deep copy. A missed mutation throws because the shared shape is frozen. This is sharing, not a cache. CloneForEngine, EnsureOwnShape, ResetShape and CreateEmptyLike are on the archived branch only.

Measured. Profile on the heaviest Sena word: Word clone 36% to 9% of wall time, Shape.CopyTo 28.6% to 13%, ReplayOnto 21.7% to 6.5%. Peak working set 11 GB to 6 GB. Wall-time attribution is unreliable on that machine. Counts are unchanged by construction.

What a revival must do first. The disposition says to reopen only if memory becomes the constraint, because the aliasing rules are subtle. Re-measure with a gloss, allomorph-index and feature-structure signature, since the old parity signature checked only morph counts.

Record: https://github.com/sillsdev/machine/blob/2eba2f502338705a4d529d7034cecb6303db7f35/docs/optimizations/shape-sharing-on-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).