Local variables support

[tetsuo-cpp]

Summary

Add Forth-style local variables to make complex algorithms readable and writable.

Motivation

Matmul tiling involves many named quantities (row, col, tile index, accumulator, etc.) that are impractical to manage on a raw stack. Local variables let you name values instead of juggling stack positions.

Design considerations

Standard Forth locals syntax options:

\ ANS Forth locals
: foo { a b c -- }  a b + c * ;

\ VALUE/TO style  
0 VALUE acc
10 TO acc

Implementation notes

• Locals can be lowered to memref.alloca slots or MLIR SSA values
• Parser needs to handle { ... -- } or VALUE/TO syntax
• Scope is per-word (locals are valid until ;)
• On GPU, locals should map to registers (scalar allocas that LLVM promotes to registers)

Priority

Important — significantly improves ergonomics for any non-trivial kernel.