ci: move the TinyGo gate to 0.42.0

[mparrett]

tinygo-wasi-build installs TinyGo 0.41.1, released 2026-04-22. 0.42.0 has been out since 2026-09-01, so the gate has been validating against a toolchain one release behind for the better part of a month. A pinned version is the right call for a gate — this is a bump, not a switch to latest.

What changes for us

0.42.0 carries #5401, which implements file-level //go:linkname. That removes one of the three link failures on native darwin:

0.41.1                                      0.42.0
readline term_bsd.go  _syscall.Syscall6     readline term_bsd.go  _syscall.Syscall6
x/sys    zsyscall_*   syscall_syscall       (gone)
pkg/rt/term.go        _syscall.Syscall      pkg/rt/term.go        _syscall.Syscall

The remaining two are tinygo-org/tinygo#4794, still open, so native macOS still does not link and docs/guide/tinygo.md stays accurate. #547's planned Darwin work is the thing this affects: the x/sys half of that workaround is already unnecessary on 0.42.0.

Verification

On darwin/arm64 with 0.42.0, against the stdgo lg-runtime as oracle:

• tinygo build -target=wasi ./cmd/lg-runtime succeeds, and the module boots under wasmtime past the reflect shim — the two steps this job runs.
• A 69-case differential corpus (integer arithmetic and shifts, hashing, floats, ordering, records, protocols, multimethods, transients, macros, channels, exceptions) is identical to both the stdgo runtime and to 0.41.1. No semantic change from the LLVM 20.1.1 → 22.1.4 jump.

Two numbers worth having on record, both from a single local build rather than a controlled measurement:

• the wasi module grows about 3%, 9,379,333 → 9,676,053 bytes
• the build ran in roughly half the wall time, 113s → 47s

CI on linux/amd64 is the real check for both; I measured on darwin/arm64.

Not included

0.43 will carry tinygo-org/tinygo#5612, which fixes the //go:cgo_import_dynamic trampolines that made x/sys calls segfault at runtime on darwin even once they linked. That is the other half of the #547 story and worth a look when 0.43 ships, but it is not in any release yet.

🤖 Generated with Claude Code

[nooga]

Clean, minimal TinyGo gate bump (0.41.1 -> 0.42.0) with verification data (differential corpus match, wasm size/build-time deltas). No concerns.