add: byte stream support.

[akeit0]

This PR makes files.lua pass.

But need new ILuaByteStream inteface.

[Copilot]

Pull request overview

This PR adds byte-stream support to Lua file loading so loadfile/LoadFileAsync can distinguish binary vs text chunks, and expands the test suite around BOM handling, load modes, I/O buffering, and read sequencing.

Changes:

• Introduces ILuaByteStream and makes LuaStream implement it, enabling byte-based reads.
• Updates LuaStateExtensions.LoadFileAsync and LuaState.Load(ReadOnlySpan<byte>) to enforce mode (b/t) and support binary chunk loading.
• Adds new NUnit tests for BOM stripping, mode rejection behavior, _ENV handling, io.read sequencing, and line buffering.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/Lua.Tests/LuaTests.cs	Adjusts files.lua patching to account for BOM handling and skips unsupported binary/string.dump coverage.
tests/Lua.Tests/LoadFileModeTests.cs	Adds tests asserting LoadFileAsync rejects chunks when mode disallows their type.
tests/Lua.Tests/LoadFileBomTests.cs	Adds tests ensuring UTF-8 BOM at start of file is ignored by loadfile.
tests/Lua.Tests/LoadEnvironmentTests.cs	Adds tests for explicit nil environments via load/loadfile.
tests/Lua.Tests/IoReadSequenceTests.cs	Adds regression test for io.read returning earlier results even when a later fixed read hits EOF.
tests/Lua.Tests/IoBufferingTests.cs	Adds test for line-buffered writes only becoming visible after newline.
src/Lua/LuaStateExtensions.cs	Updates LoadFileAsync to optionally use byte-stream reads and enforce mode rules.
src/Lua/LuaState.cs	Enforces mode rules in Load(ReadOnlySpan<byte>) for binary/text chunks.
src/Lua/IO/LuaStream.cs	Implements ILuaByteStream and updates buffering logic for setvbuf("line") / "no".
src/Lua/IO/ILuaByteStream.cs	Adds the new byte-stream interface.

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