feat(config,diagnostics): explicit transport in DeviceConfig, vendor-neutral diagnostics

DeviceConfig now carries an optional transport (serial/udp/tcp) instead of
the ESPE-only use_udp bool. Plugins validate it in create_driver_instance:
a fixed-transport driver configured with the wrong transport fails open()
with InvalidConfig (via InvalidConfigDriver — the plugin ABI forbids
returning nullptr) rather than silently ignoring the setting. Selectable
drivers (ESPE) switch TCP/UDP through the same field. config.json
load/save round-trips "transport" for every transport, including serial,
and migrates legacy use_udp:true entries.

Diagnostics drops the per-vendor accessors (espe_fault, rplidar_fault,
monitor_fault, sick_error, pollution_*, contamination_*, manipulation) for
one common shape: a list of DiagnosticIssue{severity, code, detail} with
cross-vendor codes, plus a raw map of vendor passthrough values and
to_json() for hosts that prefer a string. Vendor bit decoding now lives in
one place (decode_diagnostics); has_fault/has_warning/healthy keep their
meaning, so is_ready()/wait_ready() are unchanged.

Also: README regains the model/protocol and ExtraInfo tables lost in the
lidarlib->xlidar refactor (verified against current code), and the empty
xlocd/ tree left by a stray sync run is gone.

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

plugins/common/plugin_helpers.hpp

@@ -8,18 +8,65 @@
 #include <cerrno>
 #include <cmath>
 #include <cstdint>
+#include <cstdio>
 #include <cstring>
 #include <fcntl.h>
 #include <limits>
 #include <netinet/in.h>
 #include <netinet/tcp.h>
+#include <string>
 #include <sys/select.h>
 #include <sys/socket.h>
+#include <utility>
 
 namespace xlidar {
 
 inline constexpr float kDeg2Rad = 3.14159265358979323846f / 180.f;
 
+// True when the requested DeviceConfig::transport is one this driver can
+// serve: unset always matches (driver default); otherwise the declared
+// transport, or — for transport-selectable drivers — either side of the
+// TCP/UDP pair.
+inline bool transport_supported(const DriverInfo& info, const DeviceConfig& cfg) {
+    if (!cfg.transport || *cfg.transport == info.transport) return true;
+    if (info.transport_selectable)
+        return (*cfg.transport == Transport::Udp && info.transport == Transport::Tcp) ||
+               (*cfg.transport == Transport::Tcp && info.transport == Transport::Udp);
+    return false;
+}
+
+// Stand-in returned by create_driver_instance() when a structurally valid
+// config still can't be served (e.g. transport mismatch): the plugin ABI
+// forbids returning nullptr there, so the error surfaces from open() as
+// InvalidConfig instead of the setting being silently ignored.
+class InvalidConfigDriver : public LidarDriverInterface {
+public:
+    InvalidConfigDriver(DriverInfo info, std::string reason)
+        : info_(std::move(info)), reason_(std::move(reason)) {}
+
+    DriverInfo get_driver_info() const override { return info_; }
+    ErrorCode open() override {
+        std::fprintf(stderr, "[xlidar] %s: %s\n", info_.driver_id.c_str(), reason_.c_str());
+        return set_error(ErrorCode::InvalidConfig);
+    }
+    void close() override {}
+    bool recv_scan(ScanResult&, int) override {
+        set_error(ErrorCode::NotOpen);
+        return false;
+    }
+    void set_scan_callback(ScanCallback) override {}
+    bool spin_once() override {
+        set_error(ErrorCode::NotOpen);
+        return false;
+    }
+    const char* detected_model() const override { return info_.model.c_str(); }
+    bool is_open() const override { return false; }
+
+private:
+    DriverInfo  info_;
+    std::string reason_;
+};
+
 // Remap a finished scan's angular window onto [min_deg, max_deg]. Only
 // angle_min/angle_max/angle_increment are rewritten; points are untouched.
 inline void remap_scan_window(LaserScan& scan, float min_deg, float max_deg) {