The proxy can obtain a public certificate, and asks staging by default

Work breakdown 1.4. The mesh's own authority certifies internal names
and always did; a name reachable from outside needs one the world
already trusts, and there was no ACME anywhere in this repository.

Uses acme/autocert from x/crypto, which was already a dependency — one
indirect addition (x/net, for idna) and no new direct one.

Three things worth more than the feature:

**Staging is the default** (novox/hq 04-ISSUES/004). Production issuance
is rate-limited per domain and per account and does not replenish
quickly. Defaulting to production would leave the safe path depending on
remembering to opt out, on exactly the work most likely to iterate. A
staging certificate is trusted by no browser, so the mistake announces
itself on the first request rather than a fortnight later.

**A certificate is only asked for on a name the mesh routes here.**
Without that policy, anything that can reach the port and send a name
triggers an order for it — a scan becomes a stream of failed orders
against the account's rate limit, and the proxy looks healthy
throughout. What it may certify is what it was told to serve.

**A private issuer is trusted by naming a file, never by skipping
verification.** Skip would still apply on the day this points at a
public issuer, and nothing would say so.

TLS is opt-in: without TLS_LISTEN the proxy serves plain HTTP exactly as
before, which is what an internal-only mesh wants. With it and no cache,
it refuses rather than defaulting — every restart would otherwise order
new certificates, silently, until the rate limit says it does not.

go.sum

@@ -22,6 +22,8 @@ go.uber.org/goleak v1.3.0 h1:2K3zAYmnTNqV73imy9J1T3WC+gmCePx2hEGkimedGto=
 go.uber.org/goleak v1.3.0/go.mod h1:CoHD4mav9JJNrW/WLlf7HGZPjdw8EucARQHekz1X6bE=
 golang.org/x/crypto v0.55.0 h1:+KWHjbgOaAQ66dh/YlkZKHlz9ZUlq61AFirAR9ntP8M=
 golang.org/x/crypto v0.55.0/go.mod h1:uq0V9dE/fzQuJtbnL+2EhWOE63vo164FY8xqEnV9xis=
+golang.org/x/net v0.57.0 h1:K5+3DljvIuDG9/Jv9rvyMywYNFCQ9RSUY6OOTTkT+tE=
+golang.org/x/net v0.57.0/go.mod h1:KpXc8iv+r3XplLAG/f7Jsf9RPszJzdR0f58q9vGOuEU=
 golang.org/x/sync v0.22.0 h1:SZjpbeLmrCk4xhRSZFNZW5gFUeCeFgjekvI/+gfScek=
 golang.org/x/sync v0.22.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=
 golang.org/x/sys v0.47.0 h1:o7XGOvZQCADBQQ4Y7VNq2dRWQR7JmOUW8Kxx4ZsNgWs=