version: preserve semantic version separators

[pucedoteth]

SemVer separates pre-release identifiers and build metadata with periods, but semanticAlphabet omits the period.

String() uses normalizeVerString as a validator rather than a sanitizer — it only appends the metadata when the string round-trips unchanged:

if appBuild != "" {
    build := normalizeVerString(appBuild)
    if build == appBuild {
        version = fmt.Sprintf("%s+%s", version, build)
    }
}

So a dot-separated build stamp is not mangled, it is dropped from the version entirely, as if it had never been set. Adding a case to TestString shows it on master:

--- FAIL: TestString
    version_test.go:42: with-dotted-build: expected 0.22.2+exp.sha.5114f85, got 0.22.2

This is reachable rather than latent. appBuild is documented as settable at build time via -ldflags, and SemVer build metadata is dot-separated — exp.sha.5114f85 is an example from the spec itself. The same applies to AppPreRelease: a value like beta.rc1 would be dropped rather than reported.

The existing with-out-of-spec-build case (012_abc, underscore) still passes, so genuinely invalid metadata is still rejected.

btcd, which this code came from, fixed its copy in 65db493 ("version: preserve semantic version separators", btcsuite/btcd#2578).

go build ./..., go test ./version/, gofmt and go vet are clean.

One thing I noticed but left alone to keep the diff to the fix: the comment above appBuild says -X main.appBuild, but this copy lives in package version, so the flag is really -X github.com/gcash/bchd/version.appBuild. It was accurate before the code moved out of main.

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Disclosure: written with AI assistance; the analysis, fix and test were verified locally.