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>

include/param_prj.h

@@ -126,9 +126,9 @@
 #define CAT_COMM     "Communication"
 
 // SKUDAK customization suffix: bump on every Skudak-side change so the OpenInverter
-// web UI shows a distinct version (e.g. "4=1.19.R-S1") and we can visually confirm
+// web UI shows a distinct version (e.g. "4=1.20.R-S3") and we can visually confirm
 // the right firmware is flashed. Match the stm32-sine -S<N> convention.
-#define VERSTR STRINGIFY(4=VER-S1)
+#define VERSTR STRINGIFY(4=VER-S3)
 
 /***** enums ******/
 

src/charger.cpp

@@ -85,9 +85,15 @@ bool CheckTimeout()
 bool CheckDelay()
 {
    uint32_t now = rtc_get_counter_val();
-   uint32_t start = Param::GetInt(Param::timedly) * 60;
+   // Upstream uses uint32_t here, which wraps a negative timedly (the param's
+   // documented -1 "no delay" sentinel) into a ~4 billion-second timeout that
+   // never expires — leaving the state machine wedged in WaitStart forever.
+   // Use signed math for the short-circuit so timedly <= 0 still means
+   // "start immediately." The cast on the elapsed-comparison side is only
+   // reached when start > 0, so it can't reinterpret a negative as huge.
+   int start = Param::GetInt(Param::timedly) * 60;
 
-   return start <= 0 || (now - startTime) > start;
+   return start <= 0 || (now - startTime) > (uint32_t)start;
 }