ci: add a nightly compat matrix that verifies the declared Go floor

[Episkey-G]

What

Adds .github/workflows/compat.yml — a scheduled compatibility matrix over Go 1.13 / 1.16 / 1.21 / 1.25.

Why

ci.yml only gates on 1.25. If generated code ever starts using a newer language or stdlib feature, the go 1.13 line in go.mod would silently stop being true and nobody would notice.

This workflow runs on schedule + workflow_dispatch only — never on pull_request or push — so it adds no checks to codegen PRs. Compatibility is a patrol result: when it goes red a human should look, but it must not participate in deciding whether a PR can merge.

The floor is measured, not copied from go.mod

Ran the whole repo in the official golang:<v> images (linux/amd64):

version	go build ./...	go vet ./...	make ci-syntax	make test-cov
1.13.15	0	0	0	0
1.16.15	0	0	0	—
1.21.13	0	0	0	—

ci-syntax includes gofmtcheck.sh, and gofmt changed doc-comment layout in 1.19 — an older gofmt could plausibly reject modern formatting and make the matrix red on day one. Checked explicitly: gofmt -l (excluding vendor) reports 0 files on all three versions.

1.16 and 1.21 are kept as intermediate points because both changed build behaviour (-mod=readonly becoming the default; toolchain auto-switching), so a future breakage can be localised.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Episkey-G]

按仓库维护者判断撤回：兼容矩阵是纯巡检，不参与门禁判定，也没人会去盯一个非阻断的定时任务。声明的下界还成不成立是另一个问题，需要时手工跑一次即可，不必为它常驻一条每天运行的 workflow。

留档一下这次量出来的数字，省得以后有人重新做一遍。在官方 golang 镜像里跑整仓：1.13.15 / 1.16.15 / 1.21.13 三个版本的 go build ./...、go vet ./...、make ci-syntax 全部 exit 0，1.13 上 make test-cov 也是 exit 0。另外特意查过 gofmt 的跨版本漂移——ci-syntax 里含 gofmtcheck，而 gofmt 在 1.19 改过 doc comment 排版——gofmt -l（排除 vendor）在这三个版本上命中数都是 0。也就是说截至今天，go.mod 里那行 go 1.13 是成立的。

PR#548 的 PR 门禁不受影响，已在 master 上正常运行。