Si12T: default to original firmware sensitivity + add config get/set API

[GOB52]

Summary

Two related changes for the Si12T head-touch sensor:

1. Default the sensor to the same sensitivity the original StackChan firmware
   uses (SI12T_Type_Low / SI12T_Sensitivity_Level_3, register 0x33)
   instead of the current SI12T_Type_High / SI12T_Sensitivity_Level_4
   (0xCC).
2. Expose a small runtime configuration API on TouchSensor_Class
   (sensitivity, response/debounce, sleep, reference recalibration), with
   matching getters that read the value back from the chip.

Motivation

This change set was created to help improve / mitigate a false-trigger problem
on the Si12T head-touch sensor: it sometimes reports a touch / swipe gesture
while nothing is touching it.

This is not tied to a particular sensitivity setting. It occurs both with
the sensitivity the original StackChan firmware ships (Type_Low / Level_3)
and with StackChan-BSP's current default — it has been observed on my own device
and reported by other users.

Concrete example: on a StackChan the avatar spontaneously plays its "head-pet"
reaction — the facial expression changes and the neck servo moves — even though
no one touched the head, because the gesture recognizer saw a spurious non-zero
getIntensities() reading cross the swipe threshold.

This PR does not by itself claim to eliminate the issue; it adds the building
blocks to investigate and mitigate it:

• setResponseCycles() — RTC debounce; a touch must persist for cycles + 2
  scan cycles before it counts, rejecting single-sample spikes.
• recalibrate() — re-establish the idle baseline after an external disturbance
  (e.g. power-rail / I2C activity) that can skew it.
• setSensitivity() and the getters — tune and read back the configuration at
  runtime.

It also aligns the default sensitivity with the original firmware
(Type_Low / Level_3). Note that this alignment is for parity with the
firmware, not a fix for the false trigger, which occurs at that setting too.

Default sensitivity

The original StackChan firmware initialises the Si12T at Type_Low /
Level_3 (firmware/main/hal/hal_head_touch.cpp):

si12t_setup(si12t, SI12T_TYPE_LOW, SI12T_SENSITIVITY_LEVEL_3);

StackChan-BSP currently defaults to the more sensitive Type_High / Level_4.
This PR aligns the BSP default with the firmware so out-of-the-box behaviour
matches the reference implementation.

New configuration API

TouchSensor_Class gains setters and getters. Getters read the value back from
the chip registers and fall back to the last-set value only if the I2C read
fails.

void setSensitivity(SI12T_Type type, SI12T_Sensitivity_Level level);
void setResponseCycles(uint8_t cycles);   // RTC[2:0]; response cycle = cycles + 2 (debounce)
void setSleep(bool enable);               // low-power sleep scan mode
void recalibrate();                       // force a reference (baseline) update

SI12T_Type              getSensitivityType();
SI12T_Sensitivity_Level getSensitivityLevel();
uint8_t                 getResponseCycles();
bool                    getSleep();

Underlying Si12T helpers were added to back these:

• SI12T_Set_Config(ms, ftc, ilc, rtc) composes the CFIG register
  (0x08 = MS | FTC[1:0] | ILC[1:0] | RTC[2:0]); set_Ctrl1() now routes
  through it so the previously hard-coded 0x22 is the explicit default.
• SI12T_Reset_Reference() pulses the Ref_rst registers (0x0A/0x0B) to
  force a baseline update (datasheet 12.2.4).
• SI12T_Read_Sensitivity() / SI12T_Read_Config() / SI12T_Read_Ctrl()
  return the raw register byte (or -1 on read failure) for the getters.
• SI12T_Set_Sensitivity() now caches the configured type/level (used as the
  getter fallback).

Register fields and bit layouts follow the Si12T datasheet (section 12).

Notes

• getSleep() reflects the actual chip state. Because set_Ctrl2() enables the
  low-power sleep scan mode during normal operation, getSleep() returns true
  after begin(); setSleep(false) switches to the active (fast) mode. This is
  the chip's own semantics.
• No change to the touch read/gesture path; this PR only touches configuration.

Testing

• With the new Type_Low / Level_3 default: idle intensities stay clean
  ([0,0,0]) and intentional head touches/swipes are still detected.
• Setters/getters compile and link; getters decode the expected register fields.