fix: implement ABS as a builtin

[ghaith]

Summary

Calling ABS with an unsigned argument (USINT/UINT/UDINT/ULINT) resolved to a monomorph the standard library does not provide. The compiler emitted a bare external declaration, the shared object carried an undefined symbol, and the call crashed at runtime.

ABS is now a compiler builtin covering the whole ANY_NUM domain:

• signed integers lower to the llvm.abs intrinsic with is_int_min_poison = false, so ABS(INT_MIN) wraps to INT_MIN exactly like the previous library implementation
• floats lower to llvm.fabs
• unsigned arguments are returned unchanged and report the new warning E150 ("ABS on a value of unsigned type 'UINT' has no effect") — an unsigned ABS call is almost always a mistake, e.g. an argument expression that already underflowed

Notes for review

• The builtin coexists with the generic ABS declaration that standard library includes still ship (same generic-overload exemption ADD already relies on). Index::get_builtin_function now checks every POU registered under a name instead of only the first, so builtin codegen dispatch no longer depends on index insertion order.
• Llvm::get_intrinsic_declaration is extracted from build_lifetime_marker and reused by the ABS codegen.
• Explicit calls to ABS__DINT etc. still resolve to the library monomorphs and link as before (covered by lit).
• The stdlib ABS integration tests moved to tests/lit/single/builtin/abs_signed.st / abs_floats.st since ABS is no longer implemented by the library; abs_unsigned.st and abs_unsigned_warning.st cover the unsigned runtime behavior and the E150 diagnostics end-to-end.

Release/1.0.x counterpart with a minimal patch-lane approach: #1850

Test plan

• cargo test --workspace
• ./scripts/build.sh --lit (default + -Onone)
• new codegen inline snapshots, validator unit tests, and four lit tests

🤖 Generated with Claude Code

[github-actions]

Build Artifacts

🐧 Linux

Artifact	Link	Size
deb-x86_64	Download	38.4 MB
schema	Download	0.0 MB
stdlib	Download	32.4 MB
plc-x86_64	Download	43.5 MB
deb-aarch64	Download	30.8 MB
plc-aarch64	Download	43.4 MB

From workflow run

🪟 Windows

Artifact	Link	Size
stdlib.lib	Download	4.0 MB
stdlib.dll	Download	0.1 MB
plc.exe	Download	38.3 MB

From workflow run