feat: Go Tier 1 + gopls Tier 2 backend

[SimplyLiz]

Summary

• Add Go variant to Language enum with .go extension detection and tree-sitter-go = "0.21" grammar
• Implement go_symbols (functions, methods, struct/interface/alias types, consts), go_occurrences, and go_calls (including selector expressions) in SymbolExtractor
• Add gopls.rs Tier 2 backend (spawns gopls, standard LSP documentSymbol + hover flow, confidence 90)
• Wire gopls into Tier2Manager with .go dispatch, lazy init, and graceful degradation
• 9 new unit tests covering extraction, export rules, and CPG call edges

Test plan

• cargo test -p lip --lib — 179 passed, 0 failed