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/service/s6.cpp

@@ -0,0 +1,47 @@
+#include "kappa/service/service.hpp"
+
+#include <format>
+#include <sstream>
+
+namespace kappa::service {
+
+std::string generate_s6_service(const ServiceSpec& spec) {
+    std::ostringstream out;
+
+    // --- type file content ---
+    std::string type_content;
+    if (spec.type == "oneshot") {
+        type_content = "oneshot";
+    } else {
+        // "longrun", "notify", and anything else map to longrun in s6
+        type_content = "longrun";
+    }
+
+    // --- run file content ---
+    std::ostringstream run;
+    run << "#!/bin/execlineb -P\n";
+    run << "# Generated by kappa — do not edit manually\n";
+    run << std::format("# s6 service: {}\n", spec.name);
+
+    if (!spec.working_dir.empty()) {
+        run << std::format("cd {}\n", spec.working_dir);
+    }
+    for (const auto& [key, value] : spec.env) {
+        run << std::format("export {} \"{}\"\n", key, value);
+    }
+    if (!spec.user.empty()) {
+        run << std::format("s6-setuidgid {}\n", spec.user);
+    }
+    run << spec.exec << "\n";
+
+    // --- combined output ---
+    out << std::format("# --- s6 service directory: {} ---\n", spec.name);
+    out << "# file: type\n";
+    out << type_content << "\n";
+    out << "# file: run\n";
+    out << run.str();
+
+    return out.str();
+}
+
+} // namespace kappa::service