[levno-710]

Adds the official Lua 5.1 tests alongside the existing Lua and Luau cases. The runner isolates each case, supports selecting cases, presets, runtimes, and seeds, and saves failure logs and generated scripts for reproduction.

The tests exposed several compatibility issues. This PR fixes:

• Lost nil arguments and return values, including forwarding through functions, methods, and the outer chunk wrapper.
• Parenthesized calls and varargs incorrectly expanding into multiple values.
• Method lookup happening after argument evaluation.
• Incorrect dependencies when compiling nested function declarations.
• Numeric loops failing to coerce valid string inputs or preserve their initial values, limits, and steps.
• Generic loops discarding additional iterator results.
• Indexed assignments losing their original table or key when later expressions changed local variables.
• Function environments set before a call being ignored, including environment inheritance by nested functions.
• Dead VM registers retaining objects and preventing garbage collection.
• Merged assignments exceeding Lua’s temporary register budget.
• VM dispatch selecting the wrong block for consecutive IDs or overwriting dispatch entries when IDs overlapped list indices.
• Number serialization and mutation changing non-finite values or overflowing hexadecimal representations.

Adds regression checks for these fixes and documents compatibility limitations, including legacy implicit arg variables and replacing a function’s environment while it is running.