all: apply godoc [Name] link conventions across comments

Every Go-identifier reference in // and /* */ comments now uses
godoc's [Name] linking syntax so pkg.go.dev and `go doc` render
them as clickable cross-references. No behaviour change.

Pattern applied across the tree:
  In-package         [Foo], [Foo.Bar]
  Cross-package      [pkg.Foo], [pkg.Foo.Bar]
  Stdlib             [netip.Prefix], [errors.Is], [context.Context]
  Tailscale          [tailcfg.MapResponse], [tailcfg.Node.CapMap],
                     [tailcfg.NodeAttrSuggestExitNode]

Skip rules:
  - File:line refs left as plain text
  - HuJSON wire keys inside backtick raw strings untouched
  - ACL/policy syntax tokens (tag:foo, autogroup:self, ...) not Go
    symbols, left as plain text
  - JSON/OIDC wire keys, gorm tags, RFC IPv6 placeholders, markdown
    link tags, decorative dividers — all left as-is

hscontrol/servertest/ephemeral_test.go

@@ -23,7 +23,7 @@ import (
 //     channel is never closed (documented as a v3 TODO upstream).
 //     - https://github.com/hashicorp/golang-lru/blob/v2.0.7/expirable/expirable_lru.go#L78-L81
 //
-//  2. database/sql internal goroutines: Uses sync.RWMutex which is not
+//  2. database/sql internal goroutines: Uses [sync.RWMutex] which is not
 //     durably blocking in synctest, causing hangs.
 //     - https://github.com/golang/go/issues/77687 (mutex as durably blocking)
 //
@@ -77,7 +77,7 @@ func TestEphemeralNodes(t *testing.T) {
 
 		// Ensure the ephemeral node's long-poll session is fully
 		// established on the server before disconnecting. Without
-		// this, the Disconnect may cancel a PollNetMap that hasn't
+		// this, the [TestClient.Disconnect] may cancel a [controlclient.Direct.PollNetMap] that hasn't
 		// yet reached serveLongPoll, so no grace period or ephemeral
 		// GC would ever be scheduled.
 		ephemeral.WaitForPeers(t, 1, 10*time.Second)