feat: add HIL diagnostics and meter health handling

include/meter_parser.h

@@ -0,0 +1,54 @@
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+
+constexpr size_t METER_FRAME_MAX = 512;
+constexpr uint8_t METER_REQUIRED_FIELDS_MASK = 0x1f;
+
+enum class MeterFrameEvent : uint8_t {
+  None = 0,
+  Complete,
+  Timeout,
+  Overflow,
+  Resync
+};
+
+struct MeterFrameCollector {
+  char buffer[METER_FRAME_MAX + 1];
+  size_t length;
+  uint32_t last_rx_ms;
+  bool in_frame;
+};
+
+void meter_frame_collector_init(MeterFrameCollector &collector);
+MeterFrameEvent meter_frame_collector_check_timeout(MeterFrameCollector &collector,
+                                                     uint32_t now_ms,
+                                                     uint32_t timeout_ms);
+MeterFrameEvent meter_frame_collector_feed(MeterFrameCollector &collector,
+                                            char byte,
+                                            uint32_t now_ms);
+
+enum class MeterParseStatus : uint8_t {
+  Valid = 0,
+  Empty,
+  MissingTerminator,
+  LineTooLong,
+  MissingRequired,
+  InvalidMeterSeconds,
+  MalformedValue
+};
+
+struct MeterParseResult {
+  float energy_total_kwh;
+  float total_power_w;
+  float phase_power_w[3];
+  uint32_t meter_seconds;
+  uint8_t fields_mask;
+  bool meter_seconds_valid;
+  bool valid;
+  MeterParseStatus status;
+};
+
+bool meter_parser_parse(const char *frame, size_t len, MeterParseResult &result);
+const char *meter_parse_status_text(MeterParseStatus status);