PyCG drops the call edge for decorators with arguments (functools.lru_cache)

[rahlk]

Problem

At -a 2, one decorator on a function produces a call-graph edge and another on the same
function produces none. Verified on main (6f02581) with a fixture whose module-level
plain_function carries @trace('hot') and @functools.lru_cache(maxsize=128):

app -> trace(tag) ['pycg']
Account -> trace(tag) ['pycg']
deco(fn) -> wraps ['jedi', 'pycg']

functools.wraps gets an edge. functools.lru_cache — same module, same file, same decorator
position — gets none, and never appears in external_symbols either.

This was found while investigating #129. That issue closed because decoration is being made
recoverable through qualified_name rather than through edges (see
the spec, D5),
but the missing edge is a separate defect that survives that decision: it is a hole in the call
graph, not a question about how decoration is modelled.

Scope boundary

Diagnoses and fixes why this call edge is missing. Does not change how decoration is
represented (that is #128), does not add a decoration edge family, and does not re-attribute the
existing app -> trace(tag) edges — those are correct for evaluation order.

Goals

• Root-cause why lru_cache is dropped where wraps is not — written down, not just patched
• Determine whether the cause is specific to decorators-with-arguments, or affects ordinary
  calls too; if the latter, the fix belongs at the shared site, not the decorator path
• Missing edge emitted, with its target homed in external_symbols
• Regression test with a hand-written expected edge set

Caveats and known risks

• Fix the cause, not the symptom. If PyCG mishandles a broader class of calls, patching only
  the decorator path leaves every sibling caller still broken.
• PyCG is archived upstream (Nov 2023) and already carries local patches in
  semantic_analysis/pycg/pycg_analysis.py. A fix likely lands as another local patch, which is
  maintenance debt worth naming rather than absorbing silently.
• L2 contract. New edges at L2 are additive and safe for L1 ⊆ L2. Changing existing edge
  endpoints is not, and is out of scope.
• The Jedi backend may already resolve this correctly — worth checking whether the gap is
  PyCG-only before patching PyCG.

Definition of done

• The root cause is stated in the PR description, not just fixed
• @functools.lru_cache(maxsize=128) produces a call edge whose target is homed in
  external_symbols
• The expected edge set for the fixture is written by hand first, then matched exactly
• analysis.json(-a 1) ⊆ analysis.json(-a 2) still holds