add runtime bounds checking and improve array method error diagnostics

[cs01]

Summary

• Add runtime array bounds checking for numeric arrays and Uint8Arrays — out-of-bounds access now prints a clear error message ("Error: array index N out of bounds (length M)") and exits instead of segfaulting
• Convert 31 throw new Error() calls to ctx.emitError() across all array method codegen — users now get proper compiler errors with file/line info instead of ugly stack traces when they misuse array methods (wrong arg count, wrong arg type for filter/map/reduce/find/some/every/etc.)
• Add runtime helper infrastructure (__cs_bounds_check, __cs_null_check) in LLVM declarations

Notes

• Object arrays and string arrays don't have bounds checking yet — the compiler's own code has JS-style arr[0] patterns on empty arrays that need fixing first (tracked separately)
• Numeric array and Uint8Array bounds checking passes all 449 tests + self-hosting Stage 1

Test plan

• New test fixture array-bounds-check.ts verifies OOB access exits with code 1
• All existing tests pass (449 node, 448 native)
• Self-hosting Stage 1 passes

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