Remove the staticcheck GOTOOLCHAIN pin once a stable Go 1.27-capable release ships

The `Go lint, coverage, generated tables` job pins `GOTOOLCHAIN: go1.26.6` for the staticcheck step alone. It is temporary debt from the go1.27.0 stable rollover on 2026-08-20.

**Why the pin exists.** staticcheck cannot decode export data from a toolchain released after it, so when `setup-go`'s `stable` moved to go1.27.0 the job began failing on every run in this repository with:

```
-: internal error in importing "internal/byteorder" (cannot decode "internal/byteorder",
export data version 4 is greater than maximum supported version 2); please report an issue (compile)
```

Verified locally against go1.27.0 at the time:

| staticcheck | go1.27.0 |
|---|---|
| 2025.1.1 (pinned) | fails |
| 2026.1 | fails |
| 2026.2rc1 | clean |

Only a release candidate worked, and a prerelease linter inside a required merge gate seemed worse than the pin.

**Scope of the pin.** It covers the staticcheck step only. The rest of that job, and the whole `go-test` matrix across ubuntu/macos/windows, stay on `stable`, so go1.27 is genuinely exercised — the module suite, `go vet`, and the generator all pass under it. What the pin costs is that staticcheck analyzes under go1.26.6 rather than the newest toolchain.

**What to do.** When 2026.2 ships as a final release (or any later stable release supporting the then-current Go), raise the analyzer version and drop the `GOTOOLCHAIN` env in the same commit — the workflow comment says they move together. Confirm with:

```sh
cd go && go run honnef.co/go/tools/cmd/staticcheck@ ./...
```

run under the toolchain `stable` currently resolves to, with no `GOTOOLCHAIN` set.

Same rollover also required raising govulncheck from v1.1.4 to v1.7.0 (v1.1.4 panicked with `unexpected expr: *ast.KeyValueExpr` on a go1.27 source tree). That one is a real fix, not debt — it scans with the real `stable` toolchain and needs no follow-up.

Start in the workflow's “Go lint, coverage, generated tables” job and inspect the staticcheck step and its accompanying comment. After a stable staticcheck release supports the current Go toolchain, update the analyzer, remove its GOTOOLCHAIN setting, and run `cd go && go run honnef.co/go/tools/cmd/staticcheck@ ./...` under stable with no GOTOOLCHAIN set.

Written by the indexing model from the issue text.