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Fix MSVC C6262 warning: reduce ubpf_exec_ex stack usage (#752)

* Fix MSVC C6262 warning: reduce ubpf_exec_ex stack usage

Addresses issue #751 where ubpf_exec_ex exceeded MSVC's 1024-byte stack
threshold (was 1324 bytes).

Changes:
- Reduce _reg array from 16 to 11 elements (R0-R10 per RFC 9669)
- Reduce saved_registers array from 5 to 4 elements (R6-R9 callee-saved)
- Hoist dividend/divisor locals from signed div/mod switch cases
- Hoist _base_addr/_offset/_eff_addr/_ptr from BOUNDS_CHECK macros
- Hoist atomic operation locals (fetch, dest, val, res) to function scope

Estimated stack reduction: ~250-316 bytes.

Fixes #751

* Initialize hoisted variables to fix sanitizer warnings

The uninitialized hoisted variables triggered maybe-uninitialized warnings
in sanitizer builds. Initialize all hoisted locals to zero/NULL.

* Revert _reg[11] to _reg[16] for API compatibility

The ubpf_debug_fn callback signature in the public API uses registers[16],
so we must keep 16 elements in the _reg array. The other stack reduction
optimizations remain in place.

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Co-authored-by: Alan Jowett <alan.jowett@microsoft.com>

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