Add experimental Godot terminal export target

[nicodes]

Godot projects can now export to a Termcade target and play inside terminal cells. A real Godot EditorExportPlatformExtension produces .tgd packages; a headless engine runs ordinary GDScript and physics while a software renderer converts supported 2D scene nodes into the existing terminal framebuffer. No browser, graphical window or GPU readback is used.

Adds godot init, export, play and agent-facing capture with JSON results and input replay. Includes a paddle example, compatibility errors, isolated export staging, bounded frames/logs, and owned-process watchdog/cleanup. CI and arcade Release share an artifact gate that exercises the shipping archive in a real PTY.

This is an experimental local developer target: supported sprites, solid polygons/lines/rectangles and transforms, not universal Godot compatibility. Native execution requires --trusted; .tgd is separate from sandboxed .tcade and is not accepted by the existing marketplace. General UI, cameras, shaders, autoloads, audio, durable saves, mouse/controller support and OS sandboxing remain outside this implementation.

Validation: canonical make install and final make check passed, including five-platform archives, lint, race/consumer tests, real Godot export/render/input/reset, a genuinely hung simulation, deterministic replay, PNG capture and shipping-archive PTY pause/resize/exit. Non-Linux binaries were compiled, not executed.

Private design and source-bound evidence are maintained in the companion nicodes/docs branch feat/termcade-godot-target; the public README is generated from its canonical usage source.

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