Initial import: nasmon health monitor, split out of home_nas

Go service (nasmon/) plus nfpm packaging (packaging/), carried over from
the home_nas monorepo. Adds rpm packaging alongside the existing deb
build (Fedora/RHEL/Rocky dependency names via nfpm overrides, per-format
systemd unit path, dpkg+rpm-compatible maintainer scripts) and a Gitea
Actions workflow that builds both formats on every push.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/collect/mem.go

@@ -0,0 +1,77 @@
+package collect
+
+import (
+	"bufio"
+	"bytes"
+	"os"
+	"strconv"
+	"strings"
+)
+
+type Memory struct {
+	Total     uint64 `json:"total"`
+	Available uint64 `json:"available"`
+	Used      uint64 `json:"used"` // Total - Available
+	SwapTotal uint64 `json:"swap_total"`
+	SwapUsed  uint64 `json:"swap_used"`
+	ARCSize   uint64 `json:"arc_size"`
+	ARCMax    uint64 `json:"arc_max"`
+}
+
+func ReadMemory() (Memory, error) {
+	b, err := os.ReadFile("/proc/meminfo")
+	if err != nil {
+		return Memory{}, err
+	}
+	m := parseMeminfo(b)
+	if ab, err := os.ReadFile("/proc/spl/kstat/zfs/arcstats"); err == nil {
+		m.ARCSize, m.ARCMax = parseArcstats(ab)
+	}
+	return m, nil
+}
+
+func parseMeminfo(b []byte) Memory {
+	kv := map[string]uint64{}
+	sc := bufio.NewScanner(bytes.NewReader(b))
+	for sc.Scan() {
+		f := strings.Fields(sc.Text())
+		if len(f) < 2 {
+			continue
+		}
+		n, err := strconv.ParseUint(f[1], 10, 64)
+		if err != nil {
+			continue
+		}
+		kv[strings.TrimSuffix(f[0], ":")] = n * 1024
+	}
+	m := Memory{
+		Total:     kv["MemTotal"],
+		Available: kv["MemAvailable"],
+		SwapTotal: kv["SwapTotal"],
+	}
+	m.Used = m.Total - m.Available
+	m.SwapUsed = m.SwapTotal - kv["SwapFree"]
+	return m
+}
+
+// parseArcstats picks "size" and "c_max" from the kstat table (name type data).
+func parseArcstats(b []byte) (size, max uint64) {
+	sc := bufio.NewScanner(bytes.NewReader(b))
+	for sc.Scan() {
+		f := strings.Fields(sc.Text())
+		if len(f) != 3 {
+			continue
+		}
+		n, err := strconv.ParseUint(f[2], 10, 64)
+		if err != nil {
+			continue
+		}
+		switch f[0] {
+		case "size":
+			size = n
+		case "c_max":
+			max = n
+		}
+	}
+	return
+}