Add GPR write breakpoints (break when a register is written, anywhere)

New debugging primitive: break whenever any instruction writes to a
given general-purpose register (0-31), regardless of which address
executes the write. Requested for continuing the reboot.bin trace,
where the actual blocker is "what sets $s3 to this bad value", not
"what happens at a specific address" - existing address/memory
breakpoints can't express that directly.

- GPRBreakpoint (Core/Debugger/Breakpoints.h) mirrors the existing
  BreakPoint/MemCheck shape (result/condition/logFormat/hit count),
  keyed by register index instead of address/range.
- BreakpointManager keeps a u32 bitmask (bit i = register i has an
  active breakpoint) alongside the GPRBreakpoint vector, so the
  interpreter loop can test "would this write trip anything" with a
  single shift+and against a value already cached in a local.
- RunUntilDowncountZeroWithChecks (Core/MIPS/MIPSTables.cpp) computes
  the about-to-be-written register from the current instruction's
  OUT_RT/OUT_RD/OUT_RA flags (GetGPRWriteTarget()) and checks it
  against the mask, same convention as the existing memcheck handling
  right above it (checked before the instruction executes, bails via
  CORE_STEPPING_CPU without running it if tripped).
- New BreakReason::GPRBreakpoint ("cpu.gprBreakpoint") for Core_Break.
- WebSocket API: cpu.gprBreakpoint.add/update/remove/list, accepting
  either a 0-31 'register' index or a case-insensitive 'name' (e.g.
  "s3"), documented in docs/WebSocketDebugger.md.

Interpreter-only for now, deliberately.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GZq8ZtJmFY7bkX5FVkr3P9

docs/WebSocketDebugger.md

@@ -105,7 +105,7 @@ file - this is just an index.
 | Game/version | `game.reset`, `game.status`, `version` | `GameSubscriber.cpp` |
 | CPU core | `cpu.stepping`, `cpu.resume`, `cpu.status`, `cpu.getAllRegs`, `cpu.getReg`, `cpu.setReg`, `cpu.evaluate` | `CPUCoreSubscriber.cpp` |
 | Stepping | `cpu.stepInto`, `cpu.stepOver`, `cpu.stepOut`, `cpu.runUntil`, `cpu.nextHLE` | `SteppingSubscriber.cpp` |
-| Breakpoints | `cpu.breakpoint.add/update/remove/list`, `memory.breakpoint.add/update/remove/list` | `BreakpointSubscriber.cpp` |
+| Breakpoints | `cpu.breakpoint.add/update/remove/list`, `memory.breakpoint.add/update/remove/list`, `cpu.gprBreakpoint.add/update/remove/list` (break when a GPR is written to, by any instruction anywhere - interpreter-only, no effect under a JIT backend) | `BreakpointSubscriber.cpp` |
 | Memory read/write | `memory.read_u8/u16/u32`, `memory.read`, `memory.readString`, `memory.write_u8/u16/u32`, `memory.write` | `MemorySubscriber.cpp` |
 | Memory search | `memory.search` - scan a range for a `u8`/`u16`/`u32`/`float` value or a `bytes` pattern (with an optional wildcard mask), for narrowing down where an unknown value lives (Cheat Engine style) | `MemorySubscriber.cpp` |
 | Memory info/annotations | `memory.mapping`, `memory.info.config/set/list/search` | `MemoryInfoSubscriber.cpp` |