feat: receive ChargeLimit_pct from VCU on CAN 0x212 (SKUDAK-516)

Pairs with stm32-hal-vcu PR #42. Adds a runtime "vcuchglim" param that
the VCU broadcasts every 200 ms; on receipt, the param's Change()
callback translates 20-100% to the charger's udcspnt (DC voltage
setpoint) using a linear cell-voltage ramp.

Without this, the teslacharger had no path for the user's app-set
charge limit to actually limit charging — udcspnt was effectively
a compile-time setpoint.

What's added
------------

- Param::vcuchglim in include/param_prj.h:
    PARAM_ENTRY(CAT_CHARGER, vcuchglim, "%", 20, 100, 80, 23)
  - Range 20-100 (matches VCU validation)
  - Default 80% if VCU never connects
  - ID 23 (next free per the comment, bumped to 24)
  - Saveable so a flashed charger paired with a silent VCU still
    behaves predictably across reboots

- can->AddRecv(Param::vcuchglim, 0x212, 0, 8, 1) registration in
  ChargerCAN::MapMessages() — libopeninv's CanMap dispatches to the
  Change() callback on each received frame.

- Param::Change(vcuchglim) handler in src/main.cpp:
    cell_target = 3.30 + (4.15 - 3.30) * (pct - 20) / 80
    udcspnt     = cell_target * 96
  Endpoints chosen to be safe across Tesla LDU / VW MEB / Volt2:
    20%  → 3.30 V/cell × 96 = 316.8 V (deep-cycle storage floor)
    80%  → 3.94 V/cell × 96 = 378.0 V (recommended daily ceiling)
    100% → 4.15 V/cell × 96 = 398.4 V (matches existing udclim default)
  Tuneable here as constants if a particular pack needs a tighter range.

- VER bump 1.19.R → 1.20.R. Combined with the pre-existing -S1 suffix
  the OI web UI shows "4=1.20.R-S1", visually distinct from upstream
  and from the previous Skudak build.

Build
-----

text=25288, links clean. The "RWX LOAD segment" linker warning is
pre-existing on this template, unrelated. No new warnings from this
change.

Version string in the binary verified as "4=1.20.R-S1".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/chargercan.cpp

@@ -100,6 +100,13 @@ void ChargerCAN::MapMessages(CanMap* can)
    can->AddSend(Param::idc, 0x109, 24, 16, 1);
    can->AddSend(Param::opmode, 0x109, 40, 3, 5); //Set charging and connlock at once
 
+   /***** SKUDAK VCU command RX (0x212) — SKUDAK-516 user ChargeLimit *****/
+   // Byte 0: ChargeLimit_pct (0-100). Bytes 1-7 reserved 0x00.
+   // Param::Change(vcuchglim) translates % to udcspnt setpoint (see src/main.cpp).
+   // VCU broadcasts every 200 ms once teslacharger detected; charger holds last
+   // value if VCU goes silent.
+   can->AddRecv(Param::vcuchglim, 0x212, 0, 8, 1);
+
    /***** SKUDAK VCU telemetry (0x211) — SKUDAK-448 expanded charger telemetry *****/
    // Byte 0:    state (0=Off, 1=WaitStart, 2=Enable, 3=Activate, 4=Run, 5=Stop)
    // Bytes 1-2: udc (V, gain=1)