init: archlens — architecture diagram generator

Hex arch + DDD, tree-sitter parsing, Mermaid/ASCII output.
Supports Rust + Python. 92 tests. CI, diff, --check for staleness detection.

docs/adr/0002-tree-sitter-for-parsing.md

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+# 0002 — Tree-sitter for Source Code Parsing
+
+**Status:** Accepted  
+**Date:** 2026-06-16
+
+## Context
+
+Archlens needs to extract type-level information (classes, structs, traits, interfaces, enums, fields, inheritance) from Rust, C#, and Python source code. The tool must be language-agnostic in design, fast enough for CI, and memory-efficient for large codebases.
+
+## Decision
+
+Use tree-sitter as the primary parsing backend. One `tree-sitter` adapter crate with internal modules per language. Each language module defines tree-sitter S-expression queries to extract CodeElements and Relationships.
+
+**Tradeoff accepted:** Tree-sitter provides syntactic analysis only — no cross-file type resolution, no generics resolution, no resolved imports. This is sufficient for architecture diagrams where we care about structural relationships visible in the source text.
+
+Semantic resolution is a future concern, handled by a separate adapter (e.g., `roslyn-adapter` for C#) implementing the same `SourceAnalyzer` port.
+
+## Alternatives Considered
+
+- **Custom parsers/lexers per language:** Full control but enormous implementation and maintenance effort. Rejected.
+- **LSP servers:** Rich semantic info but heavy to run, hard to orchestrate in CI, each language needs its own server process. Rejected for CI use case.
+- **Native AST APIs (Roslyn, rustc_ast, Python ast):** Very accurate but each is a completely different API and ecosystem. Can't run Roslyn from Rust easily. Rejected as primary approach — viable as future specialized adapters.
+- **Regex/heuristic:** Breaks on edge cases. Not serious for a real tool.
+
+## Consequences
+
+- Single unified parsing approach across all languages
+- Adding a new language = writing tree-sitter queries (hours, not weeks)
+- No semantic type resolution — `use Foo` doesn't tell us which `Foo` across modules
+- Memory-efficient: parse trees are per-file and dropped after extraction
+- Fast: tree-sitter is incremental and optimized for performance