fix(getclass): reach non-BMP class names by encoding the query as MUTF-8

[dhtzs]

ApkHandler.get_class_dex encoded its query with .encode("utf-8"), but dex string_data_item is MUTF-8: a code point above the BMP is stored as a surrogate pair (CESU-8). For ASCII and BMP names the two encodings are byte-identical, so nothing showed — but an astral class name could never match, and the lookup reported "class not found" before decompilation ever ran.

Measured against _find_type_idx directly:

query encoding	Ll/\U00010000; (astral)	Ll/\u06e1; (BMP)	Lexample/Test;
UTF-8 (before)	not found	found	found
MUTF-8 (after)	found	found	found

Encoding the query the way the dex stores it is the whole fix. No linear fallback is needed: CESU-8 preserves UTF-16 code-unit ordering, so the binary search over the type table stays valid.

Scope

Only names containing a code point above the BMP change behaviour — ASCII and BMP names encode identically both ways, so their byte-for-byte path is untouched. I compared both encodings against each other before switching.

The encoder is mutf8 (already a dependency, used by dex_builder), a compiled extension costing ~19ms to import. It is imported only for a name that actually needs it, so ordinary lookups pay nothing, and a successful astral lookup is net-zero anyway since decompilation loads mutf8 moments later on the same path.

Verification

• python tests/run_tests.py --require-decompiler → 107 tests, OK.
• The full chain now resolves end to end: decompile_class(apk, "Ll/𐀀;") returns source, and every name from a class listing feeds back into decompile_class.
• Startup unchanged within noise: 108.40ms before → 109.27ms after (benchmark_startup.py, 9 samples; this machine reads 108–111ms with or without the change, over the 100ms Linux-CI budget either way).

Tests

Nothing in the tree covers this path today. tests/test_mutf8_strings.py is the natural home — it already owns the MUTF-8 cases — e.g. a fixture whose descriptors are sorted by UTF-16 code units with an astral entry, asserting get_class_dex resolves it. Happy to add that to this PR, or as a follow-up if you'd rather keep the change minimal.

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