feat: system-agnostic package manager — 5 inits, 2 bootloaders, parallel scheduler

Complete rewrite of kappa from a sequential build tool into a
system-agnostic package manager with runtime init switching.

Core additions:
- 5 init system backends: systemd, openrc, s6, runit, dinit
  (service file generation, enable/disable, init_paths)
- 2 bootloader backends: grub, limine (config generation, fallback entries)
- Parallel scheduler with worker pool, depth-based priority (Beta/Alpha/Zeta),
  atomic claiming, dependency tracking, deduplication, and failure propagation
- Init-switch impact analysis: only rebuild packages using ${enabledinit}
- Init-agnostic service definitions: flat NamedService blocks replace
  per-init nesting
- Package conflicts: mutual incompatibility detection in resolver
- System groups: init-agnostic group creation in DSL
- Init-agnostic hostname/timezone: direct /etc/hostname and /etc/localtime writes
- Source tarball caching at /kappa/cache/ with atomic write-then-rename
- Package recipe caching with remote fetching and version comparison
- remotes = [...] block in system config for package repositories
- Auto-fetch: rebuild resolves missing packages from remotes
- uninstall phase in package definitions
- ${enabledinit} eval variable for init-conditional builds
- Shared util module (to_lower, shell_escape)
- 54 integration tests across two shell test suites
- Comprehensive README and CONTRIBUTING guide

Bug fixes from review:
- CRITICAL: Replace std::system() with fork+execvp (command injection)
- CRITICAL: Fix scheduler deadlock on successful completion
- CRITICAL: Fix rebuild init/kernel/bootloader change detection
- HIGH: Fix path traversal via unsanitized package names in cache
- HIGH: Fix TOCTOU race in cache write with atomic rename
- HIGH: Fix formatter dropping remotes/imports blocks
- HIGH: Fix formatter stripping empty-string assert values
- HIGH: Fix formatter non-idempotent output (sorted key iteration)
- HIGH: Populate ${enabledinit} from boot.init in BuildStep
- MEDIUM: Fix data race on non-atomic scheduler stop flag
- MEDIUM: Fix compute_depths() traversal direction
- MEDIUM: Add runit to doctor supported-init warning
- MEDIUM: Extract to_lower/shell_escape to shared kappa::util
- MEDIUM: Consolidate generator declarations in headers

src/dsl/lexer.cpp

@@ -12,8 +12,9 @@ static const std::unordered_map<std::string_view, TokenType> keywords = {
     {"source",   TokenType::KwSource},
     {"depends",  TokenType::KwDepends},
     {"provides", TokenType::KwProvides},
-    {"outputs",  TokenType::KwOutputs},
-    {"features", TokenType::KwFeatures},
+    {"outputs",   TokenType::KwOutputs},
+    {"conflicts", TokenType::KwConflicts},
+    {"features",  TokenType::KwFeatures},
     {"config",   TokenType::KwConfig},
     {"const",    TokenType::KwConst},
     {"license",  TokenType::KwLicense},
@@ -29,6 +30,7 @@ static const std::unordered_map<std::string_view, TokenType> keywords = {
     {"build",    TokenType::KwBuild},
     {"check",    TokenType::KwCheck},
     {"install",  TokenType::KwInstall},
+    {"uninstall", TokenType::KwUninstall},
     {"true",     TokenType::KwTrue},
     {"false",    TokenType::KwFalse},
 };
@@ -51,6 +53,7 @@ std::string_view token_name(TokenType type) {
     case TokenType::KwDepends:  return "depends";
     case TokenType::KwProvides: return "provides";
     case TokenType::KwOutputs:  return "outputs";
+    case TokenType::KwConflicts: return "conflicts";
     case TokenType::KwFeatures: return "features";
     case TokenType::KwConfig:   return "config";
     case TokenType::KwConst:    return "const";
@@ -67,6 +70,7 @@ std::string_view token_name(TokenType type) {
     case TokenType::KwBuild:    return "build";
     case TokenType::KwCheck:    return "check";
     case TokenType::KwInstall:  return "install";
+    case TokenType::KwUninstall: return "uninstall";
     case TokenType::KwTrue:     return "true";
     case TokenType::KwFalse:    return "false";
     }