HermitCrab synthesis: 98% of Mbugwe synthesis work goes to candidates whose root lookup hits but never match the surface #607

Claude-drafted.

98% of Mbugwe's synthesis work goes to candidates whose lexical lookup hits but whose synthesis never matches the surface form. It is the largest unaddressed bucket on Mbugwe, and no sound prune is known. The same share was 86% on the private deletion variant, but that figure was measured before a port fix (fix B) that cut the variant's total work by 94.7%, so it does not describe the current port or hc.dll.

Status: open research lead, not started.

What we know (HC-Rust port, synthesis work in work steps):

• 98% on Mbugwe; the record does not give the sample or the fix state for that figure.
• 86% on the private deletion variant, before fix B (303,966,311 work steps then, 16,220,190 after).
• A lexical oracle cannot prune these candidates: the root is reachable, so lexical-reachability pruning (ledger rows 10 and 11) cannot reach them.
• The 2026-10-27 census on hc.dll attributes steps by failure reason (SurfaceFormMismatch is 27.2% of Mbugwe synthesis steps) but does not split root-reachable failures.

Not known: which rules reject these candidates, and whether a sound condition exists.

Proposed change: classify these failures by the rule that rejects them, in work steps. Do not add a prune until a sound condition is found.

Related: #534 (PR, closed, not merged), #560 (open). The failure-reason trace that would do the classification is #534's; the census harness (branch research/failure-reason-stats at af97348b) is test-only. PanGloss: sillsdev/PanGloss#109 (pointer, open), which depends on sillsdev/PanGloss#34 (open, ports #534).

Next: revive the #534 failure-reason trace, or extend the census harness, and report work steps per rejecting rule on Mbugwe.