fix: review fixes + feat: imports, assertions, merge engine, per-package overrides

Review fixes (8 blocking issues):
- Extract ParseError to shared error.hpp (ODR fix)
- Remove dead package.cpp/package.hpp + tomlplusplus dep
- Safe parse_int() helper replacing crash-prone std::stoi
- consume_string() now accepts bare numbers and idents
- line_at() fixed for post-EOF line numbers
- Subcommand validation before file read in CLI
- KwService/KwAssert/KwImport added to consume_ident()
- root partition promoted to first-class BootBlock field

New features:
- Imports: imports = [...] with recursive merge resolution
- Assertions: assert { "msg" : field op value } in both parsers
- Merge engine: resolve_package() with features/config merge + force support
- Per-package overrides: /kappa/system/builds/<name>.kap
- .gitignore: added vcpkg_installed/ and kappa binary

src/dsl/lexer.cpp

@@ -20,6 +20,8 @@ static const std::unordered_map<std::string_view, TokenType> keywords = {
     {"patches",  TokenType::KwPatches},
     {"env",      TokenType::KwEnv},
     {"service",  TokenType::KwService},
+    {"assert",   TokenType::KwAssert},
+    {"import",   TokenType::KwImport},
     {"prepare",  TokenType::KwPrepare},
     {"build",    TokenType::KwBuild},
     {"check",    TokenType::KwCheck},
@@ -53,6 +55,8 @@ std::string_view token_name(TokenType type) {
     case TokenType::KwPatches:  return "patches";
     case TokenType::KwEnv:      return "env";
     case TokenType::KwService:  return "service";
+    case TokenType::KwAssert:   return "assert";
+    case TokenType::KwImport:   return "import";
     case TokenType::KwPrepare:  return "prepare";
     case TokenType::KwBuild:    return "build";
     case TokenType::KwCheck:    return "check";