Share one call-site walker between L1 and L3 #20

Is your feature request related to a problem? Please describe.
L1 records call sites in buildCallSites (internal/syntactic_analysis/symbol_table.go). L3 has its own copy of the same walk in recordedCalls (internal/dataflow/bodynodes.go), and closures are walked twice the same way (buildInnerCallables and closureUnits). L3 pairs its calls with L1's call keys by position in that walk. A count check catches a change in the number of calls, but not a change in order with the same count.

Describe the solution you'd like
One shared walker that yields call sites and closures in recording order, used by both L1 and L3.

Describe alternatives you've considered
An order assertion on the fixtures. It catches drift only on the shapes the fixtures happen to contain.

Additional context
Found by the task review of PR #16 (Task 11).