feat: add Erlang language support (tree-sitter + erlang_ls)

[jafreck]

Summary

Add Erlang support via tree-sitter + LSP enrichment, complementing existing Elixir support in the BEAM ecosystem.

Tooling

Component	Tool	Notes
Tree-sitter grammar	tree-sitter-erlang	WhatsApp/Meta-maintained, high quality
LSP server	erlang_ls	Active, well-maintained
Alternative LSP	ELP	WhatsApp-backed, newer
SCIP indexer	None	N/A

What to implement

• Add tree-sitter-erlang as an npm dependency
• Create src/indexer/extractors/erlang.ts:
  • Extract -module(Name) as module symbol
  • Extract function_name(Args) -> Body declarations as function symbols (include arity in signature)
  • Extract -record(Name, {Fields}) as type/struct symbols
  • Extract -type and -spec declarations
  • Extract function calls as call refs — both local foo(X) and remote module:foo(X)
  • Extract -import(Module, [Fun/Arity]) and -include("header.hrl") as imports
  • Extract -behaviour(gen_server) callback implementations as relationships
• Register erlang in SourceIndexStage EXTRACTORS map and ParserPool LANG_PACKAGES
• Add erlang_ls to src/indexer/lsp/registry.ts: { command: 'erlang_ls', args: [] }
• Add file extensions .erl, .hrl to walker
• Add test fixtures in tests/fixtures/erlang/
• Add extractor tests

Language notes

• Functions are identified by Module:Function/Arity (e.g., lists:map/2)
• Pattern-matching clauses: one function name can have multiple clause bodies — use first clause's line as start, last clause's end
• Header files (.hrl) contain macros and record definitions — should be indexed
• OTP behaviours (gen_server, gen_statem, supervisor) define callback relationships

Context

Complements existing Elixir support. Erlang and Elixir share the BEAM VM and often co-exist in projects. Many Elixir projects call into Erlang libraries.