labels: add class_reader, hand-rolled JVM .class file parser

Foundation for label parsers that need structured access to .class
files inside /BDMV/JAR/<x>.jar. Replaces noak (~3KLOC dep) with a
~1000-line std-only reader.

Public API:
- ClassFile::parse(&[u8]) -> Result<ClassFile>
- ConstantPool::{get, utf8, class_name, string, integer, member_ref, iter}
- Member::code(&pool) -> Option<CodeAttribute>
- CodeAttribute::instructions() -> Instructions iterator
- Instruction::{name, operand_u8, operand_u16, cp_index}
- Opcode constants (LDC, AASTORE, NEW, GETSTATIC, INVOKESPECIAL, ...)

Spec coverage:
- Constant pool: all 17 tag types incl. Long/Double 2-slot quirk
- Modified UTF-8 incl. 0xC0 0x80 -> U+0000 special case
- Bytecode iteration with full opcode size table
- Variable-length tableswitch / lookupswitch / wide

12 unit tests cover the opcode table edge cases (padded switch tables,
wide-iinc 6-byte form), modified-UTF-8 decoder, and iterator
stop-on-truncated behaviour.

Module is currently #![allow(dead_code)] — the public API is staged
for labels::deluxe (Phases A-E bytecode walker) and a labels::dbp
refactor onto the constant-pool iterator. Tests exercise the API
in isolation. The allow comes off as those callers land.

Also fixes two pre-existing clippy lints that 1.86's stricter checks
flagged after I touched the labels module:
- src/mux/disc.rs: while-let-loop in test fixture
- tests/pass_n_size_aware_skip.rs: type_complexity in helper signature

Precommit (cargo +1.86 fmt + clippy + test) green.

src/labels/mod.rs

@@ -7,6 +7,7 @@
 //!   3. Implement `pub fn parse(reader: &mut dyn SectorReader, udf: &UdfFs) -> Option<Vec<StreamLabel>>`
 //!   4. Add `mod myformat;` below and one line to `PARSERS` array
 
+pub(crate) mod class_reader;
 mod criterion;
 mod ctrm;
 mod dbp;