[BUG] Concurrent map read/write in template render

[moritzrinow]

Hi, I've discovered a concurrency issue while rendering the same Template instance with a rate of 100/s:

fatal error: concurrent map read and map write
...
internal/runtime/maps.fatal({0x7ff7310261d5?, 0x7ff730220a6c?})
...
github.com/Notifuse/liquidgo/liquid.(*Context).FindVariable(0xc000465b00, {0xc0002e8470, 0x7}, 0x0)
        ...github.com/!notifuse/liquidgo@v0.0.0-20251124135804-bb1578ffeff3/liquid/context.go:528 +0x165
github.com/Notifuse/liquidgo/liquid.(*VariableLookup).Evaluate(0xc0002f2060, 0xc000465b00)
        ...github.com/!notifuse/liquidgo@v0.0.0-20251124135804-bb1578ffeff3/liquid/variable_lookup.go:215 +0x5b
github.com/Notifuse/liquidgo/liquid.(*Context).Evaluate(0x7ff730e6ad80?, {0x7ff730f1b2a0?, 0xc0002f2060?})
        ...github.com/!notifuse/liquidgo@v0.0.0-20251124135804-bb1578ffeff3/liquid/context.go:478 +0x45
github.com/Notifuse/liquidgo/liquid.(*Variable).Render(0xc00028bbd0, {0x7ff731223620, 0xc000465b00})
        ...github.com/!notifuse/liquidgo@v0.0.0-20251124135804-bb1578ffeff3/liquid/variable.go:352 +0x46
github.com/Notifuse/liquidgo/liquid/tags.(*AssignTag).RenderToOutputBuffer(0xc0002a6960, {0x7ff731223620, 0xc000465b00}, 0x0?)
        ...github.com/!notifuse/liquidgo@v0.0.0-20251124135804-bb1578ffeff3/liquid/tags/assign.go:53 +0x2d
github.com/Notifuse/liquidgo/liquid.(*BlockBody).renderNodeOptimized(0xc0001f3980?, {0x7ff730fa6d20, 0xc0002a6960}, {0x7ff731223620, 0xc000465b00}, 0xc0000495b0, 0x0, 0xc000465b00)
        ...github.com/!notifuse/liquidgo@v0.0.0-20251124135804-bb1578ffeff3/liquid/block_body.go:569 +0x682
github.com/Notifuse/liquidgo/liquid.(*BlockBody).RenderToOutputBuffer(0xc0002a6300, {0x7ff731223620, 0xc000465b00?}, 0xc0000495b0)
        ...github.com/!notifuse/liquidgo@v0.0.0-20251124135804-bb1578ffeff3/liquid/block_body.go:467 +0x265
github.com/Notifuse/liquidgo/liquid.(*Document).RenderToOutputBuffer(0xc000205f00?, {0x7ff731223620?, 0xc000465b00?}, 0xc000355700?)
        ...github.com/!notifuse/liquidgo@v0.0.0-20251124135804-bb1578ffeff3/liquid/document.go:126 +0x97
github.com/Notifuse/liquidgo/liquid.(*Template).Render(0xc000205f00, {0x7ff730edfc60, 0xc0002f39b0}, 0xc000355700)
        ...github.com/!notifuse/liquidgo@v0.0.0-20251124135804-bb1578ffeff3/liquid/template.go:348 +0x2f4
github.com/Notifuse/liquidgo/liquid.(*Template).RenderBang(...)