Add settable seed TEC setpoint and °C readout

Expose the Koheron CTL200 thermistor setpoint (rtset) as a settable HA
number in ohms, clamped to SEED_RT_SETPOINT_MIN/MAX_OHM. The former
read-only setpoint sensor is replaced by this control. Add a read-only
seed temperature in °C, computed in firmware from the actual thermistor
resistance via an NTC β-model (constants in config.h; update once the
DFB1550LP thermistor R25/β are confirmed).

Verified end-to-end over MQTT: set/clamp/restore of rtset and the °C
conversion (10 kΩ → 25.00 °C).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

include/config.h

@@ -31,6 +31,21 @@
 // guardrail against accidental over-drive, not a safety interlock.
 #define PUMP_CURRENT_MAX_MA   4000
 
+// Seed thermistor setpoint (CTL200 `rtset`) clamp, in ohms. The CTL200 has no
+// native °C setpoint; it regulates the seed TEC to hold the thermistor at this
+// resistance. The default operating point is 10 kΩ. The range below spans
+// roughly 39…13 °C on the β-model (a guardrail, not a diode-safe interlock —
+// narrow it if the DFB1550LP datasheet gives a tighter window).
+#define SEED_RT_SETPOINT_MIN_OHM   6000
+#define SEED_RT_SETPOINT_MAX_OHM   16000
+
+// NTC β-model for the read-only resistance→°C conversion (display only; the
+// CTL200 does its own regulation in ohms). Assumes a 10 kΩ @ 25 °C NTC with
+// β = 3435 K. Update if the seed thermistor's R25/β are known to differ.
+#define SEED_THERMISTOR_R25_OHM    10000.0f
+#define SEED_THERMISTOR_BETA_K     3435.0f
+#define SEED_THERMISTOR_T25_K      298.15f
+
 // ── Serial links to the instruments (wired in later increments) ─────
 // UART0 stays the USB console. The classic ESP32 has 3 hardware UARTs.
 //