[FractumSeraph]

A client that is still downloading the game's Custom Edition map (map sharing) no longer freezes when the host starts the game.

The bug

1. A client joins a lobby on a map it doesn't have and starts downloading it by map sharing.
2. The host starts the game. It correctly holds the client's players out ("machine Port: keep blended colours within [0, 1] #1 is still downloading the map; the others will not wait").
3. The client's download began in the lobby, so it doesn't know the game has started and asks for its player again. The host takes that as a late joiner's request and starts the machine at once ("machine Port: keep blended colours within [0, 1] #1 joins the game in progress at game tick Port: keep blended colours within [0, 1] #1").
4. The client loads a map it doesn't have yet. cache_files_precache_map_begin shows the damaged disc error (display_error_damaged_media), which stops the game for good.

Stock build-201 does the same with a build-201 host, every time the host starts during a download.

The fix

• Host (network_server_manager.c): a player asked for by a machine still downloading the map waits with the players held at the start (network_game_server_hold_requested_player). It joins with them, the machine started as a late joiner, once the download is done (network_game_server_release_map_fetchers).
• Client (network_client_manager.c): a start that arrives while the map is still downloading is put off until the map is there (network_game_client_begin_game, network_game_client_idle_pregame). This covers a host of an earlier build. A put-off start is dropped if its game is over by then (another map, or another game).
• NETCODE.md, "Map sharing": it said the host's start waits for a downloading machine, which it hasn't since version 28. It now says what happens.

Nothing sent changes, so the network version stays 28.

Checked

• Linux CI steps in Docker (tools/ci_build.py linux release, the test step's pytest list: 325 passed; test_light_storage.py).
• Web build and tools/test_web_build.py.
• Windows release build (clang-cl, MSVC 2019's libraries).
• End to end on Linux in Docker, on the loopback, the host starting the lobby game as soon as the client joined (the case that froze):

Host	Client	Before	After
This build	This build	Damaged disc error, game stopped	The host held the player ("holding a player on machine #1 until the map is downloaded"), then started it at game tick 296 once the download was done; the client played
Build-201	This build	Damaged disc error, game stopped	"the host started the game while its map is being downloaded: joining it once the map is there"; the client played once the map was there
This build	Build-201	Damaged disc error, game stopped	The host held the player; stock build-201 downloaded the map and played

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