v0.3.0 — dual backend (probe-rs + OpenOCD) + AI crash diagnosis

Dual debug backend and AI-facing crash diagnosis.

Added

• Dual backend behind a single DebugBackend trait, selected at connect:
  • probe-rs (default) — native probe-rs; full flash and RTT support.
  • openocd (experimental) — talks to an already-running openocd over the
    GDB Remote Serial Protocol (openocd_address, default 127.0.0.1:3333).
    The AI uses the same tool set for both; only connect differs.
• diagnose_fault — reads the Cortex-M SCB fault registers
  (CFSR/HFSR/MMFAR/BFAR/SHCSR/CPUID) plus PC/SP/LR in one call and returns a
  compact structured evidence bundle with the set fault bits. Reports raw
  evidence; it does not assert a root cause.
• unwind_exception — maps a crash to source lines. On the probe-rs backend it
  returns a full DWARF backtrace via probe-rs's own unwinder; on the OpenOCD
  backend it reads the Cortex-M exception stack frame and maps the faulting
  PC/caller LR. Requires an elf_path built with debug info.
• 24 MCP tools total (added diagnose_fault, unwind_exception).

Validated on real hardware

• OpenOCD backend verified end-to-end on a real ESP32-S3 (openocd-esp32):
  connect, memory read/write, halt/run/step/reset, and register reads
  (PC/SP/LR) are correct.

Notes / limitations

• OpenOCD register reads use monitor reg <name> (openocd resolves names per
  architecture), so PC/SP/LR work across ARM/Xtensa/RISC-V. Start openocd with
  gdb_memory_map disable or it may reject the GDB connection.
• diagnose_fault and unwind_exception are Cortex-M specific and do not apply
  to Xtensa (ESP32) targets. Their probe-rs paths delegate to probe-rs's tested
  DWARF/register code; they are unit-tested but not yet exercised on a physical
  Cortex-M crash.