feat(arch/binary): .PKGINFO parser

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <[email protected]>

internal/arch/binary/pkginfo.go

@@ -0,0 +1,74 @@
+package binary
+
+import (
+	"errors"
+	"strings"
+)
+
+// PKGInfo is the parsed content of an ALPM .PKGINFO file, the metadata
+// member at the root of an Arch .pkg.tar.zst package.
+type PKGInfo struct {
+	Name, Version, Desc, URL, Arch                     string
+	Depends, OptDepends, Provides, Conflicts, Replaces []string
+}
+
+// Parse parses ALPM .PKGINFO content: lines of "key = value", full-line
+// `#` comments and blank lines ignored. Multi-value keys (depend,
+// optdepend, provides, conflicts, replaces, license) accumulate across
+// lines; single-value keys (pkgname, pkgver, pkgdesc, url, arch, pkgbase)
+// take the last occurrence. pkgver is the FULL version, already in
+// epoch+pkgrel form (e.g. "1.0-1" or "1:1.0.0-1"), and is passed through
+// verbatim. Unknown keys are ignored. Malformed lines (no '=') are
+// reported through warn and skipped, not fatal.
+//
+// Parse returns an error if pkgname is missing: a .PKGINFO without a name
+// is invalid for conversion.
+func Parse(src string, warn func(string)) (*PKGInfo, error) {
+	info := &PKGInfo{}
+	haveName := false
+	for _, line := range strings.Split(src, "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		eq := strings.Index(line, "=")
+		if eq < 0 {
+			if warn != nil {
+				warn("malformed .PKGINFO line (no '='): " + line)
+			}
+			continue
+		}
+		key := strings.TrimSpace(line[:eq])
+		value := strings.TrimSpace(line[eq+1:])
+		switch key {
+		case "pkgname":
+			info.Name = value
+			haveName = true
+		case "pkgver":
+			info.Version = value
+		case "pkgdesc":
+			info.Desc = value
+		case "url":
+			info.URL = value
+		case "arch":
+			info.Arch = value
+		case "depend":
+			info.Depends = append(info.Depends, value)
+		case "optdepend":
+			info.OptDepends = append(info.OptDepends, value)
+		case "provides":
+			info.Provides = append(info.Provides, value)
+		case "conflicts":
+			info.Conflicts = append(info.Conflicts, value)
+		case "replaces":
+			info.Replaces = append(info.Replaces, value)
+		default:
+			// pkgbase, license, builddate, packager, size, xdata and any
+			// unknown keys are not represented in PKGInfo; ignore them.
+		}
+	}
+	if !haveName {
+		return nil, errors.New("pkginfo: missing pkgname")
+	}
+	return info, nil
+}