API: make keyboard key events actually reach the emulated C64; add run/frames for deterministic replay

[arekbr]

Problem 1: keyboard endpoints never pressed a key

input/keyDown / input/keyUp replied 200 but no key ever reached the emulated machine ($CB stayed $40), and the newer input/key/down / input/key/up always replied 406 because CDebugInterfaceVice::KeyboardDown/Up unconditionally returned false after queueing the task.

Two root causes:

• the queued task called raw keyboard_key_pressed(mtKeyCode), which is a silent no-op when the keyconvmap lookup misses, and even on a hit the latch goes through the randomized KEYBOARD_RAND() alarm — bad for cycle-deterministic tooling;
• the queueing path had no way to report failure, so both endpoint families lied.

Fix

KeyboardDown/Up resolve the MTKEY code through C64KeyMapGetDefault()->FindKeyCode() at queue time and pass the resolved (matrixRow, matrixCol, shift) into the task. ExecuteTask presses the matrix position using the same immediate-latch sequence the virtual keyboard view already uses (CViewC64KeyMap::SelectKey): keyboard_key_pressed_matrix + keyboard_set_latch_keyarr + c64d_keyboard_key_down_latch (negative rows go through keyboard_set_keyarr_any). Unmapped keys and missing/invalid params now return 406; successfully queued events return 202 {"queued":true}. Both endpoint families are fixed (the legacy names are also used by the MCP bridge).

Problem 2: no deterministic N-frame run

There was no API way to run an exact number of frames (external tooling had to drive VICE with -limitcycles instead of this debugger).

Fix

New <platform>/run/frames {"count":N} generalizes the existing frame-step mechanism: RunEmulationForFrames(numFrames) sets frameStepTargetFrameNumber = current + N (RunEmulationForOneFrame now delegates with N=1, so GUI/test behavior is unchanged; the completion-wait budget scales with N). The endpoint is synchronous — the reply arrives after the frames have run, carrying frameStart and the final frame counter — and requires a paused machine (406 otherwise, since computing current+N on a running machine would race the emulation thread).

Verification (macOS arm64, Xcode 16.4, build-macos.sh)

14-point WebSocket gate against the built app, including negative controls:

• run/frames {count:50} twice from pause: frame counter advances exactly +50 each time, frameStart/frame consistent with cpu/counters/read; called while running → 406
• input/keyDown {keyCode:97} → 202 {queued:true}; after run/frames {count:3} zeropage $CB reads 10 ('A' matrix code); keyUp → $CB back to 64 (no key)
• unmapped keyCode → 406; missing param → 406
• screen/snapshot (PNG + metadata) unaffected

Commit is on top of current master (af8f1d2).