Add Java symbol indexing support

[uchebnick]

Goal

Add first-class Java Tree-sitter indexing so unch can extract useful symbols from Java codebases.

Suggested coverage

• Classes, interfaces, enums, records, annotations
• Constructors and methods
• Public/static signatures where available
• Package-qualified names when possible
• Javadoc and nearby comments

Acceptance criteria

• Java files are parsed through Tree-sitter instead of fallback text prefixes.
• Tests cover representative class/interface/enum/method examples.
• Search results include stable kind/name/qualified_name/signature fields.
• Unsupported or invalid Java syntax falls back safely without breaking indexing.