feat: DSL parser, eval system, and project structure

- Hand-written lexer + recursive-descent parser for .kap package definitions
- Package constructs: const, license, provides, outputs, patches, depends
  (version-constrained, feature-gated, output-targeted), features (enabled/
  forced/flag), config files (default/replace/merge with cfg interpolation),
  env (?= soft-set), prepare/build/check/install phases
- Evaluator: recursive  resolver for prefix, jobopts, destdir, userargs,
  cfg.*, feature.*, package vars
- Comments: // single-line, /* */ multi-line
- /kappa/{bin,temp,db,system,system/builds} path layout
- Clang 22, C++23, CMake + vcpkg (tomlplusplus)
- .clangd + .clang-tidy configured

include/kappa/eval/vars.hpp

@@ -0,0 +1,37 @@
+#pragma once
+
+#include "kappa/dsl/ast.hpp"
+
+#include <string>
+#include <string_view>
+#include <unordered_map>
+
+namespace kappa::eval {
+
+enum class VarKind {
+    Unknown,
+    Builtin,    // ${prefix}, ${jobs}, ${jobopts}, ${userargs}
+    Package,    // ${version}, ${name}, ${source}
+    Config,     // ${cfg.key}
+    Feature,    // ${feature.key} → expanded flag or ""
+};
+
+struct ResolvedVar {
+    std::string value;
+    VarKind     kind = VarKind::Unknown;
+};
+
+struct Scope {
+    std::unordered_map<std::string, std::string> builtins;
+    std::unordered_map<std::string, std::string> config;
+    std::unordered_map<std::string, dsl::FeatureDef> features;
+    std::unordered_map<std::string, std::string> package;
+};
+
+Scope make_default_scope();
+
+ResolvedVar resolve(std::string_view name, const Scope& scope);
+
+std::string interpolate(std::string_view input, const Scope& scope);
+
+} // namespace kappa::eval