Scan the LAN, bounded to configured subnets (#257)

internal/netscan/ discovers hosts on the network Maven is configured to look at:
a TCP-connect scan (net.DialTimeout, no raw sockets, no privileges) plus a read
of the kernel's ARP cache. Wired as a read-only query source, "network", so
"какие устройства в сети?" is answered by a scan instead of by whatever old note
happens to be nearest.

Scanning is a read, but an unbounded scanner on a home LAN is noisy and easy to
point somewhere it should not go, so the package is built around four bounds:

  - Scan takes NO target argument. The range comes from the config block and
    from nowhere else, so there is no exported way to scan an arbitrary prefix
    and nothing an utterance, the router, or a scanned host says can retarget
    it. That is asserted directly: the test watches every address handed to the
    dialer and fails if one falls outside the configured prefix. The ARP cache —
    the one input the network itself populates — is filtered to the configured
    range for the same reason.
  - Every configured CIDR must be private (RFC1918 / CGNAT / link-local) and no
    larger than 1024 addresses. 8.8.8.0/24, 0.0.0.0/0 and 10.0.0.0/8 are refused
    at config load, not after the packets have left.
  - Rate-limited to a configured connections-per-second across the whole scan,
    so it looks like background traffic rather than a portscan.
  - Bounded in total by MaxHosts, a per-connection timeout, a 20s turn budget
    and the context; a canceled scan stops dialing immediately.

Off unless configured: dark without "enabled": true, and applyDefaults
normalises a disabled block to nil. deploy/mavend.json carries it disabled.

BLUETOOTH IS NOT SHIPPED, AND IS BLOCKED, NOT SKIPPED. The plan's other half
(internal/bluetooth/, RSSI presence probes) needs a bluez stack that is not
here: bluetoothctl and hcitool are not installed, bluetoothd is not installed,
the bluetooth unit is inactive, and org.bluez is not on the system bus. hci0
exists as a kernel device and nothing can talk to it. The docker deploy is
further away still — it would need host networking, the D-Bus system socket
passed in, and CAP_NET_ADMIN. Writing an exec wrapper around a binary that does
not exist, against an output format nothing here can produce, would be a guess
dressed as a feature. It needs a decision about privileging the container before
any of it is worth writing.

Vikunja #257

internal/config/config.go

@@ -25,6 +25,7 @@ import (
 	"github.com/kami/maven/internal/delivery/telegramsink"
 	"github.com/kami/maven/internal/mcp"
 	"github.com/kami/maven/internal/morning"
+	"github.com/kami/maven/internal/netscan"
 	"github.com/kami/maven/internal/smarthome"
 	"github.com/kami/maven/internal/update"
 	"github.com/robfig/cron/v3"
@@ -243,6 +244,11 @@ type Config struct {
 	// disabled ⇒ Maven neither reads the house nor touches it, and no house row
 	// exists in the act allowlist. See SmartHomeConfig.
 	SmartHome *SmartHomeConfig `json:"smarthome,omitempty"`
+
+	// NetScan — the LAN scanner (Vikunja #257). nil / absent / disabled ⇒
+	// Maven never puts a packet on the network looking for hosts. See
+	// NetScanConfig.
+	NetScan *NetScanConfig `json:"netscan,omitempty"`
 }
 
 // MCPConfig — the MCP client block. Servers are dark until one has
@@ -324,6 +330,51 @@ func (c *Config) SmartHomeClient() (smarthome.Config, bool) {
 	}, true
 }
 
+// NetScanConfig — the LAN scanner block (Vikunja #257). Dark until
+// `"enabled": true`.
+//
+// The important field is Subnets, and it is the ONLY source of a scan target.
+// Nothing an utterance, a router or a scanned host says can widen or move the
+// range: internal/netscan.Scanner.Scan takes no target argument at all. Each
+// subnet must be private and no larger than netscan.MaxPrefixHosts addresses
+// (a /22), enforced at config load rather than at the first spoken scan.
+type NetScanConfig struct {
+	// Subnets — CIDRs to scan, "192.168.1.0/24".
+	Subnets []string `json:"subnets,omitempty"`
+
+	// Ports — TCP ports to try per host. Empty ⇒ 22, 80, 443, 8080.
+	Ports []int `json:"ports,omitempty"`
+
+	// Timeout — per-connection budget. 0 ⇒ 400ms.
+	Timeout Duration `json:"timeout,omitempty"`
+
+	// Rate — connections per second across the whole scan. 0 ⇒ 50. Low on
+	// purpose: a scan should look like background traffic, not a portscan.
+	Rate int `json:"rate,omitempty"`
+
+	// MaxHosts — cap on addresses probed per scan. 0 ⇒ 256.
+	MaxHosts int `json:"max_hosts,omitempty"`
+
+	// Enabled — false (the default) keeps a written block dark.
+	Enabled bool `json:"enabled,omitempty"`
+}
+
+// NetScanner maps the config block onto the netscan package's own type.
+// ok=false when absent or disabled, so validation and daemon wiring cannot
+// drift on the mapping.
+func (c *Config) NetScanner() (netscan.Config, bool) {
+	if c.NetScan == nil || !c.NetScan.Enabled {
+		return netscan.Config{}, false
+	}
+	return netscan.Config{
+		Subnets:  c.NetScan.Subnets,
+		Ports:    c.NetScan.Ports,
+		Timeout:  time.Duration(c.NetScan.Timeout),
+		Rate:     c.NetScan.Rate,
+		MaxHosts: c.NetScan.MaxHosts,
+	}, true
+}
+
 // MCPServerConfig — one MCP server.
 type MCPServerConfig struct {
 	// Name — the local handle. It prefixes every tool this server contributes
@@ -1188,6 +1239,11 @@ func (c *Config) applyDefaults() {
 		c.SmartHome.Refresh = Duration(DefaultSmartHomeRefresh)
 	}
 
+	// Same rule for the scanner.
+	if c.NetScan != nil && !c.NetScan.Enabled {
+		c.NetScan = nil
+	}
+
 	// Same rule for the crawler: a block that neither answers on demand nor
 	// watches anything has nothing to do, so it is normalised to "off".
 	if c.Crawl != nil && !c.Crawl.OnDemand && len(c.Crawl.Watches) == 0 {
@@ -1308,6 +1364,13 @@ func (c *Config) validate() error {
 			return err
 		}
 	}
+	// A scanner pointed at the public internet, or at a /8, fails here rather
+	// than after the packets have already left.
+	if nc, ok := c.NetScanner(); ok {
+		if err := netscan.Validate(nc); err != nil {
+			return err
+		}
+	}
 	if len(c.MorningRoutines) > 0 {
 		if err := morning.Validate(morningRoutinesFromConfig(c.MorningRoutines)); err != nil {
 			return err