packet wire: payload-free arms enter the nine-leg corpus

[gafferongames]

docs/SPEC.md §4.8: "All nine backends carry payload-free arms on the packet wire" and "A payload-free arm is the tag alone". The only suite was test/packet-void/ — C and C++, with its own packet-void-{c,cpp}-{read,write} controls. No schema under examples/, examples128/ or examples-wide/ declared a bare-name arm, so the construct never entered the golden-source, wire-oracle, cross-language byte-identity or golden-wire gates on Go, Rust, C#, Java, Dart, JS or Elixir. (examples/Clauses.schema's EmptyUnion is arms-of-empty-types, which §4.8 distinguishes.)

What lands

examples/VoidArms.schema — the shared nine-leg corpus (SCHEMAS := $(wildcard examples/*.schema), so it enters generation and the golden pins with no Makefile edit):

• union VoidSignal { ping; pulse VoidPulse; level VoidLevel; ack } — two payload-free arms bracketing two typed ones, so an emitter that special-cases the first or the last arm is caught.
• type VoidSignalPacket carries it behind a 3-bit lead, so the tag is not byte aligned: a union field, a second union field selecting the other bare arm, and a counted array of unions.

Its own file, not an arm on Wire.schema's ProbeShape. That union's 2-bit tag has exactly one out-of-set value, 3, and all nine legs use it as the read-refusal negative control. An arm there would make it legal and take the teeth out of nine controls. VoidSignal has four arms, so its tag is 3 bits for [0, 4] and 5..7 stay out of set.

Per-leg byte identity

No compiler change was needed: every emitter already carries the construct, and each writes the tag in 3 bits and nothing else for ping and ack — the arm costing exactly what tag 0 costs, with the C++ reference as the oracle.

leg	generated	bare arm	tag width	VoidSignalMaxBits
cpp (oracle)	generated/cpp/VoidArms{,Wire}.h	tag only	3	19
c	generated/c/VoidArms{,Wire}.h	tag only	3	19
go	generated/go/VoidArms.go	tag only	3	19
rust	generated/rust/src/voidarms.rs	tag only	3	19
cs	generated/cs/VoidArms.cs	tag only	3	19
java	generated/java/VoidArms.java	tag only	3	19
dart	generated/dart/VoidArms.dart	tag only	3	19
js	generated/js/VoidArms{,Flat}.js	tag only	3	19
elixir	generated/elixir/VoidArms.ex	tag only	3	19

MaxBits = 19 = 3 tag bits + the largest arm (VoidPulse, 16) in every leg — identical across all nine, which is the width half of byte identity.

Goldens

go test ./internal/goldens -update re-pinned. The hunks are only the new construct plus the unit's protocol id: the new unit joins package example, so every file's id moves 0x8656ae68c06b97a7 → 0xcaa884d7062cccc8, and the C ExampleView registry gains the types and the union. testdata/wire/ is untouched — no existing fixture's bytes move.

Draft: what is NOT in this PR yet

• No runtime wire pin. The nine per-leg mains (test/main.cpp, test/c/main.c, test/go/main.go, …) are hand-written and none constructs a VoidSignalPacket yet, so there is no testdata/wire/voidarms.bin for the legs to compare against. The byte identity claimed above is over the generated codecs, not over nine executed writers. The oracle case in test/main.cpp plus the eight comparisons against its pin are the follow-on.
• No per-leg negative controls in the packet-void-c-{read,write} shape for the seven legs (make/negative-controls.json untouched).
• Legs not run here: the nine-leg make test chain needs the sibling runtime checkouts and the pinned toolchains; only the compiler-side gates (./bin/schema check examples, go test ./compiler ./internal/check ./internal/goldens, gofmt -l .) were run on this bench.

🤖 Generated with Claude Code

Follow-up 97804e2 — two red gates, found by a DeepSeek read

A DeepSeek read of this branch found two gates it left red, and 97804e2 fixes both.

• TestExportedSurfaceMovesNeitherWire/examples held the protocol id and build version as literals (0x8656ae68c06b97a7 / 0x3084078b2f7d571e) and they were not moved with the corpus. The goldens above were re-pinned; the literals, which exist precisely so a re-pin cannot silence the gate, were left behind. They now carry 0xcaa884d7062cccc8 / 0x68b43963ecc918e3 — the values this PR already pinned into testdata/golden/id.txt and testdata/golden/build-version/examples.txt. This is the allowance the gate's own comment grants: a change to a projection moves them in the same commit as the goldens. examples128 is untouched.
• TestCorpusIsCanonical failed on examples/VoidArms.schema: the two typed arms of union VoidSignal carried two spaces in the type column where schemafmt writes one. make fmt canonicalizes it. Whitespace only, so neither hash moves — verified by a rm -f bin/schema rebuild and a nine-leg regeneration of generated/, which is byte-identical.