Add /stream endpoint for real-time display via MJPEG streaming

[Copilot]

HTTP Control Server currently requires polling /screen at ~30 FPS for real-time display, creating excessive TCP handshake overhead (30+ connections/sec).

Implementation

Added /stream endpoint using multipart/x-mixed-replace (MJPEG):

• src/http_control_server.h: Added hcs_stream_callback type and hcs_set_stream_callback()
• src/http_control_server.cpp: Implemented streaming endpoint with chunked content provider
• src/main.c: Added se_hcs_stream_callback() for frame generation with JPEG encoding
• docs/HTTP_CONTROL_SERVER.md: Documented new endpoint

Key features:

• Configurable FPS (1-60, default 30) via query parameter
• JPEG encoding at 85% quality for bandwidth efficiency
• Exception handling for malformed FPS input (defaults to 30)
• Thread-safe callback execution with mutex protection

Usage

GET http://localhost:8080/stream?fps=30

Works directly in HTML without JavaScript:

<img src="http://localhost:8080/stream">

Technical Notes

Chose multipart/x-mixed-replace over WebSockets:

• Standard HTTP streaming protocol for video/images
• Native browser support in <img> tags
• Already supported by httplib's chunked content providers
• One-way streaming sufficient for this use case

Single persistent connection eliminates per-frame TCP overhead while maintaining precise frame timing via std::this_thread::sleep_for().

Original prompt

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This section details on the original issue you should resolve

<issue_title>Feature Request: Realtime display stream via websockets</issue_title>
<issue_description>Hi,

does anybody think it is possible to stream the data for drawing via a websocket connection in the headless mode? Currently I'm doing 30 /step /screen calls per second to retrieve the screen in real time. This is not a good solution because of all the handshakes. It would be pretty nice to get a continuous stream of frames to draw. Sadly I don't know if this is even possible with the architecture of the emulator. Maybe someone can help me out here.

Have a good one!</issue_description>

<agent_instructions>They are asking to real time stream the screen image from the emulator over the http_control_server, similar to the /screen command, except it would show the live display. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

@skylersaleh It should be possible with the architecture of the emulator, however I don't know how to do the web/http server side. If someone knows how this works on the web dev side, I can help them hook the rest into the emulator.

• Fixes Feature Request: Realtime display stream via websockets #484

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[github-actions]

Web build for eafb108 will be live at https://web.skyemu.app/commit/eafb108cc1a9e0a73b7054aa1ffeff8e7a1ffd73/index.html