Add 0x04_pwm_cbm: bare-metal RP2350 PWM driver

- PWM slice 4, channel B on GPIO25 (onboard LED)
- Duty cycle sweep 0-100% in 5% steps for breathing effect
- Wrap 9999 with no clock division (~650 Hz from ROSC)
- UART reports each duty step
- 1390B FLASH, 11 source files, zero warnings

drivers/0x04_pwm_cbm/.vscode/c_cpp_properties.json

@@ -0,0 +1,45 @@
+{
+    "configurations": [
+        {
+            "name": "ARM GCC",
+            "includePath": [
+                "${workspaceFolder}/Inc/**"
+            ],
+            "defines": [
+                "__GNUC__",
+                "__ARM_ARCH_8M_MAIN__",
+                "__ARMCC_VERSION"
+            ],
+            "compilerPath": "${userHome}/.pico-sdk/toolchain/14_2_Rel1/bin/arm-none-eabi-gcc",
+            "compileCommands": "${workspaceFolder}/compile_commands.json",
+            "cStandard": "c11",
+            "cppStandard": "c++17",
+            "intelliSenseMode": "gcc-arm",
+            "compilerArgs": [
+                "-mcpu=cortex-m33",
+                "-mthumb"
+            ]
+        },
+        {
+            "name": "ARM GCC (Windows)",
+            "includePath": [
+                "${workspaceFolder}/Inc/**"
+            ],
+            "defines": [
+                "__GNUC__",
+                "__ARM_ARCH_8M_MAIN__",
+                "__ARMCC_VERSION"
+            ],
+            "compilerPath": "${env:USERPROFILE}/.pico-sdk/toolchain/14_2_Rel1/bin/arm-none-eabi-gcc.exe",
+            "compileCommands": "${workspaceFolder}/compile_commands.json",
+            "cStandard": "c11",
+            "cppStandard": "c++17",
+            "intelliSenseMode": "gcc-arm",
+            "compilerArgs": [
+                "-mcpu=cortex-m33",
+                "-mthumb"
+            ]
+        }
+    ],
+    "version": 4
+}