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Fix rewrite_extern_constant_load crash on values exceeding int32 range (#1054)

* Fix gsl::narrow crash in rewrite_extern_constant_load for large values

BPF MOV imm has a 32-bit immediate field that is sign-extended to 64 bits.
When rewrite_extern_constant_load() rewrites a LDDW+LDX pair into a MOV
imm, it called gsl::narrow<int32_t>(narrowed_value) which throws
narrowing_error if the resolved 64-bit constant exceeds INT32_MAX (e.g.,
0x80000000 or larger).

Fix: check whether the value fits in a sign-extended int32 before
rewriting. If it doesn't fit, return false so the caller falls back to the
original LDDW+LDX instruction sequence.

Add regression test covering boundary values.

Extract sign-extension round-trip check into reusable fits_narrow<Narrow>(value)
template in num_safety.hpp. Update elf_extern_resolve.cpp to use it.

Signed-off-by: Michael Agun <danielagun@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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