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/0001-hexagonal-architecture-with-ddd.md

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+# 0001 — Hexagonal Architecture with DDD
+
+**Status:** Accepted  
+**Date:** 2026-06-16
+
+## Context
+
+Archlens is a language-agnostic architecture diagram generator. It needs to support multiple input languages (Rust, C#, Python), multiple output formats (Mermaid, ASCII, future: D2, interactive web), and multiple IO strategies (file, stdout). The tool must be extensible without modifying core logic.
+
+## Decision
+
+Use hexagonal architecture (ports and adapters) with DDD structuring. Workspace layout:
+
+```
+crates/
+  domain/          — zero external deps (no tracing, no rayon, nothing)
+  application/     — depends on domain only + rayon, tracing (pragmatic utility exceptions)
+  adapters/
+    tree-sitter/   — SourceAnalyzer (internal modules per language)
+    walkdir/       — FileDiscovery
+    mermaid/       — DiagramRenderer
+    ascii/         — DiagramRenderer
+    file-writer/   — OutputWriter
+    stdout-writer/ — OutputWriter
+    toml-config/   — ConfigLoader
+  presentation/    — CLI (clap), composition root, tracing-subscriber setup
+```
+
+**Dependency rules:**
+- Domain depends on nothing
+- Application depends on domain
+- Adapters depend on domain (not application)
+- Presentation depends on application + adapters (composition root)
+
+**Use cases** are generic over port traits (static dispatch), not trait objects.
+
+**Pragmatic exceptions:** `rayon` and `tracing` in application crate. These are general-purpose utilities, not external system adapters. Documented explicitly to avoid precedent creep.
+
+## Alternatives Considered
+
+- **Trait objects for DI:** Simpler type signatures but runtime dispatch overhead and less compile-time safety. Rejected — generics align better with Rust idioms and zero-cost goals.
+- **DI container (shaku):** Unnecessary ceremony for Rust. Manual wiring in presentation is explicit and sufficient.
+- **Domain with tracing dependency:** Rejected to keep domain fully pure. Application wraps domain calls in spans.
+
+## Consequences
+
+- Adding a new language = new module in tree-sitter adapter (or new adapter crate)
+- Adding a new output format = new adapter crate implementing DiagramRenderer
+- Adding a new output destination = new adapter crate implementing OutputWriter
+- Use case type signatures carry multiple generic parameters (accepted tradeoff)
+- Domain crate is testable with zero setup — no mocks for infrastructure needed