Fix storyboard decode hanging on self-referential variable definitions

[YuuLuo]

A storyboard [Variables] entry whose value re-introduces its own key causes LegacyStoryboardDecoder.decodeVariables() to loop forever, hanging the game.

Details

decodeVariables() substitutes every variable into a line, repeating until a full pass makes no change:

while (line.Contains('$'))
{
    string origLine = line;

    foreach (var v in variables)
        line = line.Replace(v.Key, v.Value);

    if (line == origLine)
        break;
}

If a variable's value contains its own key (e.g. $a=x$a), each pass replaces $a with x$a, so the line grows by one character every iteration, always still contains $, and never equals origLine — the loop never terminates. (A doubling form such as $a=$a$a instead grows the line exponentially until it throws OutOfMemoryException.)

The storyboard is decoded lazily when a beatmap is played, so a beatmap containing such a storyboard imports normally and then hangs the client the moment the map is started. The try/catch in WorkingBeatmapCache.GetStoryboard() cannot catch a non-terminating loop and there is no watchdog, so recovery requires force-killing the process.

Minimal reproducer (as a .osb, or the [Variables]/[Events] sections of a .osu):

[Variables]
$a=x$a

[Events]
$a

Fix

Substitute each defined variable a single time instead of repeatedly re-scanning the line. Storyboard variables are plain text substitutions and are not expected to reference other variables, so a single pass resolves every well-formed reference (including the existing variable-with-suffix case) while removing the re-scan that never terminates for a self-referential definition.

Added a regression test that decodes a self-referential definition and asserts the surrounding well-formed sprite still decodes correctly.

[bdach]

Did you use "AI" tools when making this PR?

[YuuLuo]

Did you use "AI" tools when making this PR?

I used translation tool to help me draft this PR.

[bdach]

matches stable