fix(validation): reject FB-level VAR_TEMP referenced from a METHOD

[ghaith]

Fixes #1745.

Summary

• A VAR_TEMP declared on a FUNCTION_BLOCK was silently miscompiled
  when read from one of that FB's METHODs: the method allocated its
  own uninitialized stack slot for the temp, the load was undef,
  and any boolean expression depending on it folded to nonsense at
  optimization levels above none.
• CODESYS rejects the same source ("unresolved reference"), so this PR
  aligns behavior: the resolver no longer leaks a method's enclosing
  VAR_TEMP into the method body, and the validator emits a focused
  diagnostic (E137) instead of the generic "unresolved reference".
• ACTIONs continue to see the FB's VAR_TEMP because they share the
  FB body's stack frame.

Changes

• src/index.rs — find_local_member_recursive filters is_temp()
  when crossing from a METHOD into its enclosing POU.
• src/validation/statement.rs — fb_temp_referenced_from_method
  helper emits E137 with a secondary location at the TEMP declaration,
  shadowing the generic unresolved-reference diagnostic for this case.
• compiler/plc_diagnostics/src/diagnostics/{diagnostics_registry.rs,error_codes/E137.md}
  — register and document E137 with both fix options (promote to
  instance member, or move the TEMP into the method).

Test plan

• cargo test --workspace — all green, no regressions.
• cargo fmt --all and cargo clippy --workspace -- -Dwarnings
  — clean.
• New validator unit tests (inline snapshots):
  • method reading FB-level VAR_TEMP → E137 with secondary location.
  • FB body + ACTION reading the same TEMP → clean.
• Manual: plc --ir on the reproducer from FB-level VAR_TEMP referenced from a METHOD lowers to an uninitialized method-local #1745 now aborts with
  E137 and emits no IR (previously it produced an undef load).

[github-actions]

Build Artifacts

🐧 Linux

Artifact	Link	Size
deb-x86_64	Download	38.5 MB
schema	Download	0.0 MB
stdlib	Download	32.4 MB
plc-x86_64	Download	43.6 MB
deb-aarch64	Download	30.9 MB
plc-aarch64	Download	43.5 MB

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🪟 Windows

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stdlib.lib	Download	4.3 MB
stdlib.dll	Download	0.1 MB
plc.exe	Download	38.5 MB

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