go.mod: require 1.27.1, and add the gofmt check this lane never had

[tannevaled]

Two things, and the second is why the first was worth doing here.

The toolchain

.github/workflows/ci.yml has pinned go-version: "1.27.1" while go.mod asked for go 1.26.4. The directive now matches the pin, which matters because 1.27 counts statements more finely: a coverage figure measured with an older toolchain is an upper bound rather than a measurement, and this repository's gate is exact 100%. Measured with 1.27.1: still exactly 100%, every function, so the gate is unaffected.

The missing check

gofmt -l . reported two files on main — under 1.26's gofmt and 1.27's alike — and nothing said so, because this lane runs go vet and no gofmt check, while every sibling repository in the fleet runs both. An unformatted tree had been green for as long as it took to notice.

The lane has the check now, in the same shape the siblings use, and the two files are formatted. One needed a judgement call rather than a mechanical rewrite: font_test.go built name-table records as w.u16(3); w.u16(1); … — six calls on one line — which gofmt splits into six lines, losing the one thing that layout was for: a record is six numbers in a fixed order and reads as a row. A variadic u16s keeps the row and satisfies gofmt, which seemed better than making the file obey the tool at the cost of what it was showing.

Verified with 1.27.1 before committing

gofmt, vet, go test -race, the exact-100% coverage gate, and the six cross-build targets the lane lists — including loong64, which is built and never executed here, so golang/go#81147 (the loong64 code-generation backport, still open at the 1.27.2 milestone) cannot affect this repository.

A measurement note for whoever runs this next: the shared build cache on this machine was being pruned concurrently, which made go test fail twice with missing cache files and once with package internal/nettrace is not in std. Neither was real; a private GOCACHE made every run reproducible.

Tagged v0.13.0 on merge: raising the go directive is a build-requirement change.

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