compat: SemVer version windows between app and server

Both sides now declare what they will talk to, and enforce it. Two axes kept
deliberately separate, because conflating them is the trap:

  protocol_version  — CAN these builds talk. The correctness axis. Below 1.0.0
                      the minor is the breaking axis, per SemVer §4.
  release window    — MAY they, per policy. [min, max), advertised in the
                      profile, overridable by the operator.

The server refuses out-of-window apps with 426 and a body naming both versions
and the accepted range; the app checks the profile in both directions before a
run rather than discovering mid-measurement that it will be refused.

Three rules that shape the rest:

  - GET /v1/profile is never gated. It is where a refused client learns which
    version it needs; gating it leaves the user with a network error instead of
    an answer, which is precisely the confusion this exists to remove.
  - An unparseable or absent version is "unknown", and is allowed. Development
    builds report "dev", and a client too old to send the header cannot be
    identified anyway.
  - Bounds sit at breaking boundaries, not at releases, so shipping a patch
    never requires editing a range. The app's server minimum is 0.4.2 for a
    stated reason: earlier multi-homed servers mis-addressed granted sends and
    the client measured 100% downstream loss that never happened.

The app's versionCode is now derived from its SemVer instead of being a second
number someone has to remember to bump.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/cmd/echolot-server/main.go

@@ -36,6 +36,7 @@ import (
 	"time"
 
 	"echo-lot.app/server/internal/canarydns"
+	"echo-lot.app/server/internal/compat"
 	"echo-lot.app/server/internal/config"
 	"echo-lot.app/server/internal/control"
 	"echo-lot.app/server/internal/dataplane"
@@ -131,6 +132,15 @@ func serve(cfg *config.Config) error {
 			"retention_days", cfg.UploadRetentionDays, "max_runs_per_device", cfg.UploadMaxRuns)
 	}
 
+	// A malformed window is fatal rather than ignored: an operator who set a restriction must
+	// not end up running without one because of a typo.
+	appRange, err := compat.ParseRange(cfg.MinAppVersion, cfg.MaxAppVersion)
+	if err != nil {
+		return fmt.Errorf("app version window: %w", err)
+	}
+	slog.Info("client compatibility", "accepts_app", appRange.String(),
+		"protocol", control.ProtocolVersion, "schema", control.SchemaVersion)
+
 	ctl := &control.Server{
 		Store: st, Sessions: sessions, Name: cfg.Name,
 		UDPPort: mustPort(firstAddr(cfg.UDPListen)), TCPPort: mustPort(firstAddr(cfg.TCPListen)),
@@ -140,6 +150,7 @@ func serve(cfg *config.Config) error {
 		BigSend:     dp.BigSend,
 		TCPRecent:   func(ip string) any { return tcpSrv.RecentFor(ip) },
 		Runs:        runStore,
+		AppRange:    appRange,
 	}
 
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)