Cross-DEX method calls not executed by VM

[haeter525]

Summary

The Dalvik VM is bound to a single DEX, so a method whose bytecode lives in a different DEX of a multi-dex APK is never executed. When a method in classes.dex invokes one defined in classes2.dex, the direct/static dispatch lookup misses the single-DEX method map and the call is silently downgraded to an external API miss (stub or no-op). dextrace run <apk> therefore only produces correct results when every method actually executed happens to live in the one DEX that was loaded.

Root cause

# src/dextrace/vm/engine.py  (_do_invoke, direct/static path)
callee_code_off = self._sig_to_codeoff.get(callee_sig)
if callee_code_off is None:
    self._handle_external_miss(callee_sig, insn, state)
    return None

self._sig_to_codeoff, self._parser, self._disasm, self._resolver, and self._hierarchy are all built from a single dex_bytes. A code_off is a byte offset valid only within its own DEX, so the engine has no way to reach a callee defined in another DEX — the lookup misses and real bytecode is skipped.

Fix

Make the VM module-aware: build one DexModule (parser, resolver, disasm, local method map) per DEX, resolve direct/static calls through a global map, and carry the current module through the execution loop and CallFrame.

# global dispatch map: sig -> (module_id, code_off), stable order (classes.dex first)
location = self._sig_to_location.get(callee_sig)
if location is None:
    self._handle_external_miss(callee_sig, insn, state)
    return None
callee_module_id, callee_code_off = location
# switch active module on invoke; restore caller's module on return

Cross-DEX virtual dispatch and inheritance additionally require merging ClassHierarchy across all DEXes (vtable values become (module_id, code_off)) — a separable follow-up.

Affected locations

File	Line(s)	Context
src/dextrace/vm/engine.py	137-162	Per-DEX state built from single dex_bytes
src/dextrace/vm/engine.py	708-711	Direct/static dispatch misses cross-DEX callee
src/dextrace/vm/engine.py	692-694	Virtual dispatch via single-DEX ClassHierarchy
src/dextrace/vm/class_hierarchy.py	71-90	Vtables built from one DEX; values are bare code_off
src/dextrace/cli/cmd_run.py	137-139	dextrace run loads only the entry DEX