server: §3.4 asymmetric grants + downtrain and big_send actions

The grant is the keystone that makes server->client sends safe: created
only by an authenticated control-plane action, bound at creation to the
session's OBSERVED data-plane source (so it can never be aimed at a third
party), and bounded by bytes, average rate and expiry. Sends stop the
moment the budget runs out, so a buggy action cannot become a flood.

Two granted actions on top of it:
- downtrain: N packets at a given size/interval toward the client, with
  seq + send-timestamp in the payload — downstream loss/reorder/jitter,
  which an upstream-only train cannot measure.
- big_send: one datagram per requested size, echoing the intended size in
  the payload — downstream MTU / black-hole evidence the client cannot
  produce for itself (only the far end can emit a large packet toward it).

Tests cover the security properties: no grant without a verified
destination, client requests clamped to server limits, byte budget stops
sending exactly, expiry refuses, and the rate ceiling throttles a burst.
Capabilities gain downtrain + big-send.

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

server/internal/session/grant_test.go

@@ -0,0 +1,94 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package session
+
+import (
+	"net/netip"
+	"testing"
+	"time"
+)
+
+func sessionWithSource(t *testing.T) *Session {
+	t.Helper()
+	m := NewManager(time.Minute)
+	s, _, err := m.New("dev", "cred", netip.MustParseAddr("127.0.0.1"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	s.NoteDataSource(netip.MustParseAddrPort("127.0.0.1:5000"))
+	return s
+}
+
+// Without an observed data-plane source there is nowhere verified to send, so no grant may exist.
+func TestNoGrantWithoutObservedSource(t *testing.T) {
+	m := NewManager(time.Minute)
+	s, _, _ := m.New("dev", "cred", netip.MustParseAddr("127.0.0.1"))
+	if g := s.NewGrant("a1", 1000, 1000, DefaultGrantLimits); g != nil {
+		t.Fatal("granted a send with no verified destination")
+	}
+}
+
+func TestGrantIsBoundToObservedSourceAndClamped(t *testing.T) {
+	s := sessionWithSource(t)
+	g := s.NewGrant("a1", 1<<40, 1<<30, DefaultGrantLimits) // absurd request
+	if g == nil {
+		t.Fatal("expected a grant")
+	}
+	if g.Dest != "127.0.0.1:5000" {
+		t.Fatalf("grant destination = %s, want the observed source", g.Dest)
+	}
+	if g.MaxBytes != DefaultGrantLimits.MaxBytes || g.MaxKbps != DefaultGrantLimits.MaxKbps {
+		t.Fatalf("client request was not clamped to server limits: %d bytes / %d kbps",
+			g.MaxBytes, g.MaxKbps)
+	}
+}
+
+// The byte budget must actually stop sending — this is the anti-amplification guarantee.
+func TestGrantStopsAtByteBudget(t *testing.T) {
+	s := sessionWithSource(t)
+	g := s.NewGrant("a1", 1000, 0, DefaultGrantLimits)
+	sent := 0
+	for i := 0; i < 100; i++ {
+		if !g.Allow(100) {
+			break
+		}
+		sent += 100
+	}
+	if sent != 1000 {
+		t.Fatalf("sent %d bytes, want exactly the 1000-byte budget", sent)
+	}
+	if g.Allow(1) {
+		t.Fatal("grant allowed a send after the budget was exhausted")
+	}
+	if g.Sent() != 1000 {
+		t.Fatalf("Sent() = %d, want 1000", g.Sent())
+	}
+}
+
+func TestExpiredGrantRefuses(t *testing.T) {
+	s := sessionWithSource(t)
+	g := s.NewGrant("a1", 10_000, 0, GrantLimits{MaxBytes: 10_000, MaxKbps: 1000, MaxHold: time.Millisecond})
+	time.Sleep(5 * time.Millisecond)
+	if g.Allow(10) {
+		t.Fatal("expired grant still allowed a send")
+	}
+}
+
+// The rate ceiling must throttle a burst that is well inside the byte budget.
+func TestGrantEnforcesRate(t *testing.T) {
+	s := sessionWithSource(t)
+	// 8 kbps = 1000 bytes/s. A burst far beyond one second's worth must be refused.
+	g := s.NewGrant("a1", 1<<20, 8, DefaultGrantLimits)
+	time.Sleep(60 * time.Millisecond) // let the rate window open a little
+	sent := 0
+	for i := 0; i < 1000; i++ {
+		if !g.Allow(100) {
+			break
+		}
+		sent += 100
+	}
+	if sent > 5000 {
+		t.Fatalf("rate limit let %d bytes through in ~60ms at 8kbps", sent)
+	}
+}