Add fullscreen display and SDL gamepad support to emulator

[maxvdec]

Motivation

• Provide a fullscreen emulator window while preserving the 320x180 logical presentation so games render with correct aspect/letterboxing.
• Add real controller support so players can use gamepads (including analog sticks and D-pad) mapped to the emulator's input bits and enable two-player controllers via hotplug.

Description

• Open the SDL window with SDL_WINDOW_FULLSCREEN and keep the existing logical presentation using SDL_SetRenderLogicalPresentation so the 320x180 framebuffer is letterboxed.
• Add a GamepadSlot struct, a gamepads vector, and lifecycle methods openAvailableGamepads, openGamepad, closeGamepad, closeAllGamepads, and gamepadPad to SdlHost in emulator/include/e16/sdl_host.h.
• Implement gamepadButtons to map analog sticks (with a dead zone), D-pad, face buttons (South/East/West/North), start/back/shoulder buttons to E16 pad bits, and wire hotplug handling and input combination in emulator/src/sdl_host.cpp.
• Update the controls dialog and README.md to document fullscreen behavior and controller button/stick mappings.

Testing

• Ran code formatting (clang-format -i) over the changed files without error.
• Attempted to configure and build the emulator with cmake -S emulator -B emulator/build && cmake --build emulator/build, but configuration failed because the environment is missing the sdl3 development package (pkg-config sdl3 not found), so a full build could not be completed.

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve gamepad player slots on disconnect

When the first of two connected gamepads is unplugged, this erase compacts the vector, while gamepadPad() treats vector index 0/1 as player 1/2. In a two-player session with a transient disconnect of player 1's controller, the remaining player 2 controller starts driving player 1 and player 2 reads as disconnected; replugging the original controller then assigns it to player 2. Keep fixed slots or replace the removed slot with an empty entry so hotplugging does not silently swap player assignments.

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