occurrences: Replace machinery-ref heuristics with JL's synthetic_ref

[aviatesk]

JuliaLowering now tags machinery references (function self-refs inside K"method" / K"function_type" / K"removable", kwsorter body forwards, optional-positional self-forwarding stubs, type self-refs in _typebody! / _equiv_typedef calls, etc.) with :synthetic_ref metadata. Replace the JETLS-side heuristic layer that approximated this with a single metadata check when recording :use occurrences.

Removed: the SkipRecording byte-range map, the
K"block" [K"function_decl" …] traversal that registered ranges, is_selffunc / is_kwsorter_func / kwerr arg-skipping, and the struct-name :local rhs-pattern check. Net ~130 lines out of occurrence-analysis.jl.

Also tighten the :local (mod=nothing) → :global alias merge for type definitions to discriminate by declaration byte_range rather than by is_internal + name, so an unrelated local Foo / let Foo = 1 sharing a name is not merged.

Regression coverage in test/analysis/test_occurrence_analysis.jl: function / macro / type definitions produce no spurious :use of their own name, and user-written refs inside machinery positions (self-recursive calls, dependent defaults in optional / keyword args and inner constructors) are still counted correctly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.27%. Comparing base (e5d3c5f) to head (87f0059).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #646      +/-   ##
==========================================
- Coverage   75.39%   75.27%   -0.12%     
==========================================
  Files          58       58              
  Lines       10915    10865      -50     
==========================================
- Hits         8229     8179      -50     
  Misses       2686     2686              

Flag	Coverage Δ
JETLS.jl	75.27% <100.00%> (-0.12%)	⬇️

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