Convert tre_ast_to_tnfa() to iteration over a tre_stack

[kevinushey]

These pull requests were generated with Claude Code, but were reviewed by me (@kevinushey) before posting. I'll respond personally to any feedback; please let me know if this is okay.

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This converts tre_ast_to_tnfa() from recursion to iteration over a tre_stack, resolving the TODO at the top of lib/tre-compile.c:

Fix tre_ast_to_tnfa() to recurse using a stack instead of recursive function calls.

Problem. The recursive implementation overflows the C stack for large patterns — recursion depth is proportional to pattern size. A catenation of 50,000 literals crashes tre_regcomp() on Windows x64 (1 MB default stack, mingw-w64 gcc); the same pattern compiles and matches correctly with this change:

PATCHED   n=50000 compile=OK exec rc=0 match=(0,50000)
UNPATCHED n=50000 CRASHED (stack overflow)

Approach.

• Reuses the compilation stack already allocated in tre_compile() rather than allocating a new one; both call sites are inside tre_compile(), where the stack is idle at those points.
• Traversal order is preserved: children are pushed right-then-left so the left child pops first, and tre_make_trans() for CATENATION/ITERATION fires before descending into children, exactly as the recursive version did.
• The counting pass call site now checks the return value, since the iterative version can fail with REG_ESPACE if the stack cannot grow (the recursive counting pass could not fail).

Testing. Built with mingw-w64 gcc 14 on Windows; verified the crash reproduction above, plus sanity checks ((a|b)+c, capture groups) and larger alternation/catenation patterns up to the existing TRE_MAX_STACK parse limit, which now fails cleanly with REG_ESPACE instead of crashing.

Provenance. A version of this fix (with a separately allocated stack) has been carried in GNU R's vendored copy of TRE since 2011 and is exercised by every ERE regex call in R.