HermitCrab analysis: one-template battery is 45% of work on a private grammar; FST-derived precheck ceiling unmeasured #604

Claude-drafted.

On a one-template grammar the template battery is 45% of parser work (a private grammar variant, v1, held list; HC-Rust work steps, before change C), and the shipped shared-slot-batch change cannot help it. An FST-derived precheck could reject slot rules before traversal, but its ceiling is not measured.

Status: open research lead, not built.

What we know (HC-Rust port, work steps; the lead is tagged as having a C# analogue):

• Battery share of parser work before change C: v1 held 45%, Mbugwe 61%, Sena 23%, Amharic 21%, private base 8%, private deletion variant 6.5%, Indonesian 0 (no templates).
• Sharing trailing slot batches (change C, ported as perf(hermitcrab): share trailing analysis template slot batches #546) saves 0 work on Amharic and every private variant (one template each) and on Indonesian (no templates), so there is nothing to share.
• After fix A, v1's held list still costs 105.8M work units, 45% of it in the one-template battery.
• The nearest shipped check is the edge-segment prefilter (ledger row 22). It tests only a rule's first and last segments and was judged on traversals. This lead derives its condition from the whole compiled matcher and is judged in work steps, since a traversal charges per arc.

Not known: the ceiling, meaning the work spent in zero-match traversals on single-template grammars.

Proposed change: instrument zero-match traversal work in steps on single-template grammars first. Build the precheck only if that share is material.

Related: #564 (open), #574 (PR, open), #599 (open), #546 (PR, open). PanGloss: sillsdev/PanGloss#106 (pointer, open). The required-slot look-ahead in sillsdev/PanGloss#25 and #66 is a different mechanism (state not checked).

Next: add a zero-match traversal work counter and report it per single-template grammar.