fix: provide builtin ABS monomorphs for unsigned types

[ghaith]

Summary

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

ABS__USINT/ABS__UINT/ABS__UDINT/ABS__ULINT are now builtins:

• the generic resolver binds e.g. ABS(UINT#1) to ABS__UINT through its normal find_pou lookup — no resolver changes needed
• their codegen returns the argument unchanged (|x| = x for unsigned values)
• each call reports 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

Signed and float arguments are untouched and still call the library implementations.

Notes for review

• Builtin validation is now dispatched under the resolved call name in addition to the flat AST name (guarded against double dispatch when they match), so the warning and the argument-count check fire for generic calls that bind to a builtin.
• generate_implementation_stubs skips builtin implementations so no dead declare @ABS__UINT lands in the IR.
• E150 matches the code used on master; the registry documents the E142..E149 gap reserved for later branches.
• One pre-existing snapshot shifted (builtin_functions_named_arguments_invalid_parameter_names): its E037 message embeds a source slice from a type declared in the concatenated <builtin> unit, sliced against the wrong file — garbage before, differently-shifted garbage now that the builtin unit contains four more declarations. Might be worth a separate issue.

Master counterpart (full builtin ABS with intrinsics): #1849

Test plan

• cargo test --workspace
• ./scripts/build.sh --lit (default + -Onone)
• new codegen inline snapshot, five validator unit tests, and two lit tests (abs_unsigned.st, abs_unsigned_warning.st)

🤖 Generated with Claude Code

[github-actions]

Build Artifacts

🪟 Windows

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

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🐧 Linux

Artifact	Link	Size
deb-x86_64	Download	10.9 MB
schema	Download	0.0 MB
stdlib	Download	33.5 MB
plc-x86_64	Download	43.5 MB
deb-aarch64	Download	5.4 MB
plc-aarch64	Download	43.3 MB

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