feat: add ppsspp_set_register (write MIPS Allegrex registers)

[dmang-dev]

Implements ppsspp_set_register — the register-write half of the completeness gap flagged on the Glama profile and tracked in #1.

What it does

Writes a single MIPS Allegrex register via PPSSPP's cpu.setReg:

• register — name as shown by ppsspp_get_registers (pc, v0, a0-a3, sp, ra, f0-f31, …).
• value — a JSON integer (common case) or a string for hex / float / special forms ("0x1F", "1.5", "nan", "-inf") — PPSSPP parses all of these.

The write counterpart to ppsspp_get_registers: redirect execution, fix a return value, patch an argument before a call. Best used while paused / stopped at a breakpoint.

Why not savestate?

The roadmap issue (#1) also listed savestate management — but that's not implementable. PPSSPP's WebSocket debugger exposes no savestate API (the subscriber set is CPUCore, Breakpoint, Disasm, Game, GPU*, HLE, Input, Memory, MemoryInfo, Replay, Stepping, ClientConfig — no savestate.*). Wrapping a fake ppsspp_save_state would silently fail, as CLAUDE.md already warns. Savestates remain PPSSPP's F1-F8 UI keybinds. The CHANGELOG documents this.

Verification

• npm run build (tsc) passes; CI matrix runs on this PR.
• cpu.setReg confirmed against PPSSPP's Core/Debugger/WebSocket/CPUCoreSubscriber.cpp (params name/value).
• No live PPSSPP in CI — recommend a quick runtime check: pause, ppsspp_set_register pc 0x08800000, then ppsspp_get_registers to confirm.

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