fix: return fresh arrays from analyzeFileFull degraded paths; add tests

The shared EMPTY_STRUCTURE static leaked the same array instances into
every degraded-path result — a caller mutating its result would corrupt
every later caller's. Replace with a helper that returns fresh arrays.

Also add the test coverage the new API was missing: analyzeFileFull
output equality vs analyzeFile + extractCallGraph, cached-parser
stability across calls, unsupported-extension fallback, mutation
isolation, and PluginRegistry delegation/fallback-to-null.