Good First Issue: Add Java language detection + symbol regex

### Description
Split from #36 — Java slice. One file.

### Files
- `peek/peek/symbols.py` — add `JAVA_CLASS_RE`
- `peek/tests/test_symbols.py`

### Acceptance
- [ ] `a.java` with `class Foo` is indexed
- [ ] `pytest -q` passes

### Time
~30 min