feat: add local variables support

[tetsuo-cpp]

Summary

• Add { a b c -- } syntax for read-only local variables in word definitions
• Reuses existing forth.pop and forth.push_value ops (pure SSA approach, no new dialect operations)
• Locals work across control flow (IF/ELSE/THEN, loops) since SSA values from the entry block dominate all blocks

Test plan

• Basic local variable binding and reference test
• Locals across IF/ELSE/THEN control flow test
• Duplicate name error test
• Full pipeline to gpu.binary test
• All 88 tests pass

Closes #8

[tetsuo-cpp]

Code Review

Clean implementation — reusing forth.pop/forth.push_value as pure SSA values is the right design. Locals map to registers on GPU for free, and entry-block SSA dominance gives you control flow support without any extra work.

Suggestions

Missing error-path tests: The code checks for param and shared memory name conflicts, but neither path has a test. Consider adding:

\ param conflict
\! kernel main
\! param DATA i64[256]
: BAD { data -- } data ;
BAD

\ shared conflict
\! kernel main
\! shared BUF i64[64]
: BAD { buf -- } buf ;
BAD

{ outside a word definition: Currently falls through to "unknown word: {" which is a confusing error. A targeted diagnostic like "local variables can only be declared inside word definitions" would be friendlier. Low priority.

Minor: Output names after -- are silently skipped without validation — { a b -- 42 } is accepted. Might be worth a code comment noting this is intentional (documentation-only, per ANS Forth).

Everything else looks good. The reverse-order pop loop, localVars.clear() cleanup, and conflict checks are all correct.