fix: C backend String functions, record type aliases, docs cleanup

- Add String.fromChar, chars, substring, toUpper, toLower, replace,
  startsWith, endsWith, join to C backend
- Fix record type alias unification by adding expand_type_alias and
  unify_with_env functions
- Update docs to reflect current implementation status
- Clean up outdated roadmap items and fix inconsistencies
- Add comprehensive language comparison document

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -120,17 +120,34 @@ fn main(): Unit with {Console} =
 
 ## Status
 
-**Current Phase: Prototype Implementation**
+**Core Language:** Complete
+- Full type system with Hindley-Milner inference
+- Pattern matching with exhaustiveness checking
+- Algebraic data types, generics, string interpolation
+- Effect system with handlers
+- Behavioral types (pure, total, idempotent, deterministic, commutative)
+- Schema evolution with version tracking
 
-The interpreter is functional with:
-- Core language (functions, closures, pattern matching)
-- Effect system (declare effects, use operations, handle with handlers)
-- Type checking with effect tracking
-- REPL for interactive development
+**Compilation Targets:**
+- Interpreter (full-featured)
+- C backend (functions, closures, pattern matching, lists, reference counting)
+- JavaScript backend (full language, browser & Node.js, DOM, TEA runtime)
+
+**Tooling:**
+- REPL with history
+- LSP server (diagnostics, hover, completions, go-to-definition)
+- Formatter (`lux fmt`)
+- Package manager (`lux pkg`)
+- Watch mode / hot reload
+
+**Standard Library:**
+- String, List, Option, Result, Math, JSON modules
+- Console, File, Http, Random, Time, Process effects
+- SQL effect (SQLite with transactions)
+- DOM effect (40+ browser operations)
 
 See:
-- [SKILLS.md](./SKILLS.md) — Language specification and implementation roadmap
-- [docs/VISION.md](./docs/VISION.md) — Problems Lux solves and development roadmap
+- [docs/ROADMAP.md](./docs/ROADMAP.md) — Development roadmap and feature status
 - [docs/OVERVIEW.md](./docs/OVERVIEW.md) — Use cases, pros/cons, complexity analysis
 
 ## Design Goals
@@ -150,20 +167,31 @@ See:
 
 ## Building
 
+### With Nix (recommended)
+
+```bash
+# Build
+nix build
+
+# Run the REPL
+nix run
+
+# Enter development shell
+nix develop
+
+# Run tests
+nix develop --command cargo test
+```
+
+### With Cargo
+
 Requires Rust 1.70+:
 
 ```bash
-# Build the interpreter
 cargo build --release
-
-# Run the REPL
-cargo run
-
-# Run a file
-cargo run -- examples/hello.lux
-
-# Run tests
-cargo test
+./target/release/lux           # REPL
+./target/release/lux file.lux  # Run a file
+cargo test                     # Tests
 ```
 
 ## Examples