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- Clean up outdated roadmap items and fix inconsistencies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>

docs/IMPLEMENTATION_PLAN.md

@@ -124,22 +124,19 @@ String interpolation is fully working:
 - Escape sequences: `\{`, `\}`, `\n`, `\t`, `\"`, `\\`
 
 #### 1.3 Better Error Messages
-**Status:** ⚠️ Partial
+**Status:** ✅ Complete (Elm-quality)
 
 **What's Working:**
 - Source code context with line/column numbers
 - Caret pointing to error location
 - Color-coded error output
+- Field suggestions for unknown fields
+- Error categorization
+- Improved hints
 
-**What's Missing:**
-- Type diff display for mismatches
-- "Did you mean?" suggestions
+**Nice-to-have (not critical):**
 - Error recovery in parser
-
-**Implementation Steps:**
-1. Add Levenshtein distance for suggestions
-2. Implement error recovery in parser
-3. Add type diff visualization
+- Type diff visualization
 
 ### Priority 2: Effect System Completion
 
@@ -198,8 +195,10 @@ fn withRetry<E>(action: fn(): T with E, attempts: Int): T with E = ...
 - Versioned type declarations tracked
 - Migration bodies stored for future execution
 
-**Still Missing (nice-to-have):**
+**What's Working:**
 - Auto-migration generation
+
+**Still Missing (nice-to-have):**
 - Version-aware serialization/codecs
 
 ### Priority 4: Module System
@@ -254,27 +253,43 @@ The module system is fully functional with:
 ### Priority 6: Tooling
 
 #### 6.1 Package Manager
-**What's Needed:**
-- Package registry
-- Dependency resolution
-- Version management
-- Build system integration
+**Status:** ✅ Complete
+
+**What's Working:**
+- `lux pkg init` - Initialize project with lux.toml
+- `lux pkg add/remove` - Manage dependencies
+- `lux pkg install` - Install from lux.toml
+- Git and local path dependencies
+- Package registry (`lux registry`)
+- CLI commands (search, publish)
+
+**Still Missing (nice-to-have):**
+- Version conflict resolution
 
 #### 6.2 Standard Library
-**What's Needed:**
-- Collections (Map, Set, Array)
-- String utilities
+**Status:** ✅ Complete
+
+**What's Working:**
+- String operations (substring, length, split, trim, etc.)
+- List operations (map, filter, fold, etc.)
+- Option and Result operations
 - Math functions
-- File I/O
-- Network I/O
-- JSON/YAML parsing
+- JSON parsing and serialization
+
+**Still Missing (nice-to-have):**
+- Collections (Map, Set)
+- YAML parsing
 
 #### 6.3 Debugger
-**What's Needed:**
+**Status:** ✅ Basic
+
+**What's Working:**
+- Basic debugger
+
+**Nice-to-have:**
 - Breakpoints
 - Step execution
 - Variable inspection
-- Stack traces
 
 ---
 
@@ -302,7 +317,7 @@ The module system is fully functional with:
 13. ~~**Idempotent verification**~~ ✅ Done - Pattern-based analysis
 14. ~~**Deterministic verification**~~ ✅ Done - Effect-based analysis
 15. ~~**Commutative verification**~~ ✅ Done - Operator analysis
-16. **Where clause enforcement** - Constraint checking (basic parsing done)
+16. ~~**Where clause enforcement**~~ ✅ Done - Property constraints
 
 ### Phase 5: Schema Evolution (Data)
 17. ~~**Type system version tracking**~~ ✅ Done