feat: add multi-DEX support for parsing APKs and directories

[komen205]

Summary

Android apps exceeding the 65K method reference limit use multiple DEX files (classes.dex, classes2.dex, ..., classesN.dex). This PR implements the Multi-DEX Aware feature from the README TODO, providing a unified view across all DEX files in an APK or directory.

• Rust: MultiDexHelper with from_directory() and from_apk() constructors, unified classes()/methods()/fields()/strings() iterators, Sourced<T> wrapper for provenance tracking
• Python: MultiDEXHelper with from_apk(), from_directory(), from_dex_files() constructors, unified iterators yielding (DexSource, item) tuples
• CLI: --apk and --dir flags for both Rust and Python CLIs

Design

Since DexHelper/DEXHelper already resolve all indices to strings, there's no need to merge index namespaces. MultiDexHelper wraps multiple DexHelper instances and flat-maps their iterators, tagging each item with its source DEX filename. Fully backward compatible — no changes to existing APIs.

Dependencies

• Rust: Optional zip crate behind apk feature flag (cargo build --features apk)
• Python: Uses stdlib zipfile — zero new dependencies

Tested against

Ticketmaster APK (10 DEX files, 82K+ classes, 50K+ methods):

$ dexparser --apk base.apk
Loaded 10 DEX file(s) from APK
  [classes.dex] (index 0)
  [classes2.dex] (index 1)
  ...
  [classes10.dex] (index 9)

CLASSES:
  [classes.dex] CLASS LCityEntity; (super: Ljava/lang/Object;)
  [classes10.dex] CLASS Lorg/xmlpull/v1/XmlPullParser; (super: Ljava/lang/Object;)
  ...

Files changed

File	What
dexparser-rs/src/multidex.rs	New — MultiDexHelper, DexSource, Sourced<T>, constructors, iterators, 7 unit tests + 1 integration test
dexparser/multidex.py	New — MultiDEXHelper, DexSource, constructors, iterators
dexparser-rs/src/error.rs	Add NoDexFiles, Io error variants
dexparser-rs/src/lib.rs	Wire up pub mod multidex + re-exports
dexparser-rs/Cargo.toml	Add optional zip dep + apk feature
dexparser-rs/src/bin/dexparser.rs	Add --apk/--dir CLI flags
dexparser/__init__.py	Export MultiDEXHelper, DexSource
dexparser/main.py	Add --apk/--dir CLI flags
README.md	Remove [TODO], add multi-DEX usage examples

Test plan

• cargo test — 28 unit tests pass (sort key, name matching, empty/single/multi DEX, directory scan)
• cargo test --features apk test_real_apk -- --ignored — integration test against real 10-DEX APK
• cargo run --bin dexparser -- -i classes.dex — single-DEX backward compat verified
• cargo run --features apk --bin dexparser -- --apk base.apk — multi-DEX APK parsing verified
• Python --apk and --dir flags functional

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