fix(firmware-zig): migrate to Zig 0.15.2 API

- build.zig: replace deprecated root_source_file with root_module + b.createModule()
  on both addExecutable and addTest; b.host → b.graph.host; point test root at
  new src/all_tests.zig to avoid main.zig's freestanding-only externs
- main.zig: convert /** */ block comment to // line comments (Zig has no block comments)
- ota.zig: rename field error → last_error ('error' is a keyword in Zig)
- sensors.zig: calculateQuality takes *const Self (never mutates); test locals → const
- src/all_tests.zig: new host-side test runner that imports all submodules with tests
  (buffer, sensors, mqtt, ota, wifi) while skipping main.zig firmware stubs

10/10 tests pass on Zig 0.15.2.

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

Author: hyperpolymath

firmware-zig/build.zig

@@ -15,12 +15,14 @@ pub fn build(b: *std.Build) void {
 
     const optimize = b.standardOptimizeOption(.{});
 
-    // Main firmware executable
+    // Main firmware executable (Zig 0.15+: use root_module)
     const exe = b.addExecutable(.{
         .name = "kaldor-bbw-firmware",
-        .root_source_file = b.path("src/main.zig"),
-        .target = target,
-        .optimize = optimize,
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/main.zig"),
+            .target = target,
+            .optimize = optimize,
+        }),
     });
 
     // Link with ESP-IDF components (when available)
@@ -30,11 +32,14 @@ pub fn build(b: *std.Build) void {
 
     b.installArtifact(exe);
 
-    // Unit tests (run on host)
+    // Unit tests (run on host — Zig 0.15+: use root_module).
+    // Uses all_tests.zig as root to avoid main.zig's freestanding-only externs.
     const unit_tests = b.addTest(.{
-        .root_source_file = b.path("src/main.zig"),
-        .target = b.host,
-        .optimize = optimize,
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/all_tests.zig"),
+            .target = b.graph.host,
+            .optimize = optimize,
+        }),
     });
 
     const run_unit_tests = b.addRunArtifact(unit_tests);

firmware-zig/src/all_tests.zig

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: PMPL-1.0-or-later
+//
+// Kaldor IIoT firmware — host-side test runner.
+//
+// Pulls in unit tests from each submodule. main.zig is excluded here because
+// it contains firmware stubs (hal.*, mqtt_client, etc.) that only compile
+// when targeting ESP32/freestanding; host tests would fail on those externs.
+
+const std = @import("std");
+
+test {
+    _ = @import("buffer.zig");
+    _ = @import("sensors.zig");
+    _ = @import("mqtt.zig");
+    _ = @import("ota.zig");
+    _ = @import("wifi.zig");
+}

firmware-zig/src/main.zig

@@ -1,18 +1,16 @@
 // SPDX-License-Identifier: PMPL-1.0-or-later
 
-/**
- * Kaldor IIoT — Back Beam Width (BBW) Sensor Firmware.
- *
- * This Zig module implements the firmware for an ESP32-based monitoring 
- * system. It is designed for high-reliability industrial environments, 
- * featuring local data buffering, watchdog enforcement, and secure telemetry.
- *
- * SUBSYSTEMS:
- * 1. SENSORS: High-frequency (100Hz) acquisition of physical metrics.
- * 2. BUFFER: Circular memory buffer to prevent data loss during WiFi outages.
- * 3. TELEMETRY: MQTT client for publishing processed metrics and alerts.
- * 4. MAINTENANCE: OTA (Over-The-Air) update listener for remote management.
- */
+// Kaldor IIoT — Back Beam Width (BBW) Sensor Firmware.
+//
+// This Zig module implements the firmware for an ESP32-based monitoring
+// system. It is designed for high-reliability industrial environments,
+// featuring local data buffering, watchdog enforcement, and secure telemetry.
+//
+// SUBSYSTEMS:
+// 1. SENSORS: High-frequency (100Hz) acquisition of physical metrics.
+// 2. BUFFER: Circular memory buffer to prevent data loss during WiFi outages.
+// 3. TELEMETRY: MQTT client for publishing processed metrics and alerts.
+// 4. MAINTENANCE: OTA (Over-The-Air) update listener for remote management.
 
 const std = @import("std");
 const config = @import("config.zig");

firmware-zig/src/ota.zig

@@ -34,7 +34,7 @@ pub const OtaError = enum {
 pub const OtaUpdater = struct {
     device_id: []const u8,
     state: OtaState = .idle,
-    error: OtaError = .none,
+    last_error: OtaError = .none,
     progress: u8 = 0,
     current_url: ?[]const u8 = null,
     bytes_received: usize = 0,
@@ -63,7 +63,7 @@ pub const OtaUpdater = struct {
         self.state = .downloading;
         self.progress = 0;
         self.bytes_received = 0;
-        self.error = .none;
+        self.last_error =.none;
 
         if (builtin.os.tag == .freestanding) {
             // ESP32: Start OTA update
@@ -97,7 +97,7 @@ pub const OtaUpdater = struct {
                     } else if (status < 0) {
                         // Error
                         self.state = .error_state;
-                        self.error = .download_failed;
+                        self.last_error =.download_failed;
                         hal.print("OTA: Download failed\n");
                     }
                 } else {
@@ -115,7 +115,7 @@ pub const OtaUpdater = struct {
                         hal.print("OTA: Verification passed, installing...\n");
                     } else {
                         self.state = .error_state;
-                        self.error = .verify_failed;
+                        self.last_error =.verify_failed;
                         hal.print("OTA: Verification failed\n");
                     }
                 } else {
@@ -131,7 +131,7 @@ pub const OtaUpdater = struct {
                         hal.restart();
                     } else {
                         self.state = .error_state;
-                        self.error = .install_failed;
+                        self.last_error =.install_failed;
                     }
                 } else {
                     self.state = .complete;
@@ -164,7 +164,7 @@ pub const OtaUpdater = struct {
 
     /// Get last error
     pub fn getError(self: *const Self) OtaError {
-        return self.error;
+        return self.last_error;
     }
 
     /// Check if update is in progress

firmware-zig/src/sensors.zig

@@ -236,7 +236,7 @@ pub const SensorManager = struct {
     }
 
     /// Calculate signal quality based on distance
-    fn calculateQuality(self: *Self, distance: f32) u8 {
+    fn calculateQuality(self: *const Self, distance: f32) u8 {
         _ = self;
 
         // Quality degrades with distance and noise
@@ -263,14 +263,14 @@ pub const SensorManager = struct {
 // ============================================================================
 
 test "SensorManager initialization" {
-    var sm = SensorManager.init();
+    const sm = SensorManager.init();
     // begin() would fail without hardware, but init() should work
     try std.testing.expect(!sm.ultrasonic_initialized);
     try std.testing.expect(!sm.dht_initialized);
 }
 
 test "quality calculation" {
-    var sm = SensorManager.init();
+    const sm = SensorManager.init();
     try std.testing.expectEqual(@as(u8, 0), sm.calculateQuality(10));
     try std.testing.expectEqual(@as(u8, 95), sm.calculateQuality(150));
     try std.testing.expectEqual(@as(u8, 0), sm.calculateQuality(500));