feat: add launch module (manifest, interpolation, process exec)

- manifest: Manifest struct (files, launch config, server info), LoadManifest, LibraryFiles/ModFiles filters
- launch: Game struct (Prepare/BuildCommand/Start lifecycle)
  - Prepare: download manifest, resolve Java, sync files with SHA-1, soft-delete unknown mods
  - BuildCommand: classpath assembly, variable interpolation (, , etc.), authlib-injector injection
  - Start: execute the assembled command
- interpolate:  template replacement
- cleanupUnknownMods: moves untracked mods to mods_backup/

Co-Authored-By: OWL <noreply@anthropic.com>

internal/launch/manifest.go

@@ -0,0 +1,75 @@
+// package launch handles Minecraft process launching.
+package launch
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+)
+
+// Manifest describes the contents of a modpack as provided by the backend.
+type Manifest struct {
+	MinecraftVersion string         `json:"minecraft_version"`
+	JavaVersion      int            `json:"java_version"`
+	ServerInfo       ServerInfo     `json:"server_info"`
+	Files            []ManifestFile `json:"files"`
+	Launch            LaunchConfig   `json:"launch"`
+}
+
+// ServerInfo is the game server address injected into servers.dat.
+type ServerInfo struct {
+	Name string `json:"name"`
+	IP   string `json:"ip"`
+}
+
+// ManifestFile is a single file entry with SHA-1 verification.
+type ManifestFile struct {
+	Path string `json:"path"`
+	Hash string `json:"hash"`
+	Size int64  `json:"size"`
+	URL  string `json:"url"`
+}
+
+// LaunchConfig contains the JVM and game arguments template.
+type LaunchConfig struct {
+	MainClass       string   `json:"mainClass"`
+	JVMArgs         []string `json:"jvmArgs"`
+	GameArgs        []string `json:"gameArgs"`
+	NativeLibs      []string `json:"nativeLibs"`
+	AuthLibInjector string   `json:"authLibInjector"` // URL to authlib-injector.jar
+}
+
+// LoadManifest reads and parses a manifest.json from disk.
+func LoadManifest(path string) (*Manifest, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("reading manifest %s: %w", path, err)
+	}
+	var m Manifest
+	if err := json.Unmarshal(data, &m); err != nil {
+		return nil, fmt.Errorf("parsing manifest %s: %w", path, err)
+	}
+	return &m, nil
+}
+
+// LibraryFiles returns the subset of Files that are .jar libraries (classpath).
+func (m *Manifest) LibraryFiles() []ManifestFile {
+	var libs []ManifestFile
+	for _, f := range m.Files {
+		if len(f.Path) > 4 && f.Path[len(f.Path)-4:] == ".jar" {
+			libs = append(libs, f)
+		}
+	}
+	return libs
+}
+
+// ModFiles returns the subset of Files that go into the mods/ directory.
+func (m *Manifest) ModFiles() []ManifestFile {
+	var mods []ManifestFile
+	for _, f := range m.Files {
+		if len(f.Path) > 5 && f.Path[:5] == "mods/" {
+			mods = append(mods, f)
+		}
+	}
+	return mods
+}