V2.3.26 — env-sensor diagnostics

Make a flaky SHT45 (or any silently-failing env sensor) visible in the logs.

1. sht45_read()'s two silent I²C error paths now log ESP_LOGW mirroring the init path's verbosity. Triggered on a bench unit whose SHT45 ACK'd at init then on every cycle reported H=0.00 % silently → cascade quietly fell through to BMP390.
2. env_sensor_read() gained an optional (char *raw_log, size_t cap) tail so the caller sees per-sensor reads alongside the fused result. main.c cycle log line now reads e.g.: SHT45: T=18.86°C H=0.00%, BMP390: T=18.88°C P=1026.60hPa SHT45+BMP390: T=18.86°C H=0.00% P=1026.60hPa.

No I²C transaction changes, same fallback cascade, OTA-safe from V2.3.25.