Runtime bounds checking and safe defaults

[gregwinn]

Runtime Bounds Checking and Safe Defaults

Effort: Small

Several runtime functions crash on edge cases instead of returning errors.

Issues

• `String.slice(str, start, len)` — crashes if start/len out of range (should return empty string)
• `Map.get(key, map)` — crashes if map is not a map (should return error tuple)
• `List.first([])` — should return nil, not crash
• `List.last([])` — should return nil, not crash
• `Enum.reduce([], fn)` — should handle empty list gracefully
• Map field access `user.name` — crashes with badkey if field doesn't exist (should return nil)

Solution

• Add guard clauses and fallback returns to all runtime functions
• Map field access in codegen: use `maps:get(Key, Map, nil)` instead of `maps:get(Key, Map)`
• Document which functions can return nil vs raise