feat: implement §9 file transfer over dedicated binary DataChannels

File chunks travel over a per-transfer "f:<xid>" WebRTC DataChannel —
direct DTLS-encrypted P2P, the anchor never sees file data. Once the
initial handshake is done the anchor can disappear and transfers continue.

Key design choices:
- Receiver sends "ok" on the file DC before sender streams, eliminating
  a race where tiny files could be fully sent/closed before the receiver's
  OnMessage handler is registered (open-race §6 analogue for data DCs).
- Auto-accept: receiver accepts every incoming offer immediately.
- Download dir: per-network at <data-dir>/downloads-<netid>/.
- Backpressure: bufferedAmountLowThreshold to avoid overwhelming the DC.
- SHA-256 verified on receive; mismatches emit EvtError and discard temp file.
- IPC: send_file {peer_id, path} → offers the named file from share dir.
- EvtFileComplete {transfer_id, path} emitted on success.

IPC command: {"type":"send_file","peer_id":"<hex>","path":"<filename>"}

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test-network.sh

@@ -114,6 +114,26 @@ pretty() {
                 echo -e "${color}${BOLD}[${label}]${RESET} ${BOLD}💬 #${room}${RESET} ${DIM}<${from}…>${RESET} ${body}"
             fi
             ;;
+        incoming_file)
+            local name size xid
+            name=$(echo "$line" | jq -r '.offer.name // "?"' 2>/dev/null)
+            size=$(echo "$line" | jq -r '.offer.size // 0' 2>/dev/null)
+            xid=$(echo "$line" | jq -r '.offer.xid[:8] // "?"' 2>/dev/null)
+            echo -e "${color}${BOLD}[${label}]${RESET} ${YELLOW}↓ incoming_file${RESET} ${BOLD}${name}${RESET} (${size}B) xid=${DIM}${xid}…${RESET}"
+            ;;
+        file_progress)
+            local xid rx total
+            xid=$(echo "$line" | jq -r '.transfer_id[:8] // "?"' 2>/dev/null)
+            rx=$(echo "$line" | jq -r '.bytes_received // 0' 2>/dev/null)
+            total=$(echo "$line" | jq -r '.total_bytes // 0' 2>/dev/null)
+            echo -e "${color}${BOLD}[${label}]${RESET} ${DIM}file_progress${RESET} xid=${xid}… ${rx}/${total}B"
+            ;;
+        file_complete)
+            local xid path
+            xid=$(echo "$line" | jq -r '.transfer_id[:8] // "?"' 2>/dev/null)
+            path=$(echo "$line" | jq -r '.path // "?"' 2>/dev/null)
+            echo -e "${color}${BOLD}[${label}]${RESET} ${GREEN}✓ file_complete${RESET} xid=${DIM}${xid}…${RESET} → ${BOLD}${path}${RESET}"
+            ;;
         error)
             local msg
             msg=$(echo "$line" | jq -r '.error_message // .' 2>/dev/null)
@@ -364,6 +384,40 @@ for peer_ipc in "$ALICE_IPC" "$BOB_IPC" "$CHARLIE_IPC"; do
 done
 sleep 0.5
 
+# ── file transfer ────────────────────────────────────────────────────────────
+echo ""
+echo -e "${DIM}────────────────────────────────────────────────────────${RESET}"
+echo -e "file transfer (alice → bob: notes.txt)"
+echo -e "${DIM}────────────────────────────────────────────────────────${RESET}"
+
+ipc "$ALICE_IPC" "$(jq -cn --arg peer "$BOB_ID" --arg path "notes.txt" \
+    '{"type":"send_file","peer_id":$peer,"path":$path}')"
+
+# Wait up to 10s for bob to receive the file (glob over downloads-* dir)
+received=0
+for i in $(seq 1 50); do
+    sleep 0.2
+    if ls "$DATA_ROOT/bob/downloads-"*/notes.txt 2>/dev/null | grep -q .; then
+        received=1; break
+    fi
+done
+
+if [ "$received" = "1" ]; then
+    rx_file=$(ls "$DATA_ROOT/bob/downloads-"*/notes.txt 2>/dev/null | head -1)
+    orig_sha=$(sha256sum "$DATA_ROOT/alice/share/notes.txt" | awk '{print $1}')
+    rx_sha=$(sha256sum "$rx_file" | awk '{print $1}')
+    if [ "$orig_sha" = "$rx_sha" ]; then
+        echo -e "  ${GREEN}✓ notes.txt received by bob, sha256 matches${RESET}"
+    else
+        echo -e "  ${RED}✗ notes.txt received but sha256 mismatch!${RESET}"
+        echo -e "    orig: $orig_sha"
+        echo -e "    got:  $rx_sha"
+    fi
+else
+    echo -e "  ${RED}✗ notes.txt not received by bob within 10s${RESET}"
+fi
+sleep 0.5
+
 # ── leave ─────────────────────────────────────────────────────────────────────
 echo ""
 echo -e "${DIM}────────────────────────────────────────────────────────${RESET}"