H3/H4/H5: zone-integrity invariants

H3+H4 — Zone SOA invariant. After parsing, loadRRs enforces:
exactly one SOA, owned by the zone apex. Catches three failure modes
with a single guard:

  - Missing SOA (H4): a malformed line earlier in the file may have
    tripped miekg/dns's ZoneParser into dropping records without
    reporting an error via parser.Err(). If the SOA went missing, we
    refuse rather than treat the partial parse as authoritative.

  - Multiple SOAs (H3): zone files with accidental duplicate SOA
    records produce inconsistent zone state visible to AXFR clients.
    The old code's first-match SOA-bump would silently propagate the
    inconsistency. Now we refuse.

  - Non-apex SOA (H3): an SOA whose owner doesn't match the zone
    origin is either a parse error or a hand-edit mistake; bumping
    it would leave the real apex unchanged. Now we refuse.

assertSingleApexSOA returns a descriptive error so the failure mode
is actionable from logs alone.

H5 — MaxUint32 guard in bumpSerial. The old "+1 defensive advance"
branch would wrap to 0 if soa.Serial == MaxUint32, and downstream
secondaries per RFC 1982 §3.2 treat 0-after-MaxUint32 as "older"
(they refuse to AXFR and the zone goes dark). Now we explicitly check
and refuse with a loud message; operator must reset the serial
manually. Practical reach is zero for our deployment (10000 bumps/day
× 117 years would still fit uint32) but the defensive ceiling matters
for fuzz, hand-edit, or future code-path errors.

The full RFC 1982 wraparound-aware comparison was prototyped but
removed: it broke the legacy-format migration case where a tiny
non-CalVer serial (e.g., 12345) is "more than 2^31 distant" from a
new-format serial (~2.6B), which RFC 1982 reads as "going backwards"
and would block migration. Naive `>` is correct in practice; the
MaxUint32 case is the only real failure mode worth guarding.

New tests:
- TestBumpSerial_MaxUint32_RefusesWrap
- TestLoadRRs_NoSOA_Refused
- TestLoadRRs_MultipleSOAs_Refused
- TestLoadRRs_NonApexSOA_Refused

zonefile.go

@@ -3,6 +3,7 @@ package rfc2136
 import (
 	"context"
 	"fmt"
+	"math"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -104,6 +105,15 @@ func openZoneFile(path, origin string) *zoneFile {
 // Returns (rrs, snapshot, error). The snapshot fingerprints the file
 // identity at read time so a subsequent writeIfUnchanged can detect
 // concurrent modification.
+//
+// Hamilton H4 — strict-parse validation: a single malformed line could
+// otherwise produce a partial parse where parser.Err() returns nil but
+// some records silently went missing. To catch this, we enforce a
+// post-parse invariant: exactly one SOA RR, and that SOA's name equals
+// the configured zone origin. A zone file that's been partially eaten
+// by the parser usually loses its SOA along the way — checking SOA
+// presence catches both H4 (silent truncation) and H3 (multi-SOA or
+// wrong-apex SOA) with a single guard.
 func (z *zoneFile) loadRRs() ([]dns.RR, fileSnapshot, error) {
 	f, err := os.Open(z.Path)
 	if err != nil {
@@ -130,9 +140,45 @@ func (z *zoneFile) loadRRs() ([]dns.RR, fileSnapshot, error) {
 	if len(rrs) == 0 {
 		return nil, snap, fmt.Errorf("%s: zero RRs parsed", z.Path)
 	}
+
+	// H3/H4 invariant: exactly one SOA, anchored at the zone origin.
+	// Refuse to operate on a zone file whose SOA structure is wrong —
+	// any subsequent bumpSerial or write would compound the damage.
+	if err := assertSingleApexSOA(rrs, z.Origin); err != nil {
+		return nil, snap, fmt.Errorf("zone %s integrity check failed: %w", z.Path, err)
+	}
+
 	return rrs, snap, nil
 }
 
+// assertSingleApexSOA enforces that rrs contains exactly one SOA and
+// that its owner matches the zone origin. Returns an error otherwise.
+// This is the H3+H4 zone-integrity invariant.
+func assertSingleApexSOA(rrs []dns.RR, origin string) error {
+	origin = canon(origin)
+	var soas []*dns.SOA
+	for _, rr := range rrs {
+		if s, ok := rr.(*dns.SOA); ok {
+			soas = append(soas, s)
+		}
+	}
+	switch len(soas) {
+	case 0:
+		return fmt.Errorf("no SOA record found (expected one at %q)", origin)
+	case 1:
+		if canon(soas[0].Hdr.Name) != origin {
+			return fmt.Errorf("SOA owner is %q, expected zone apex %q", soas[0].Hdr.Name, origin)
+		}
+		return nil
+	default:
+		names := make([]string, len(soas))
+		for i, s := range soas {
+			names[i] = s.Hdr.Name
+		}
+		return fmt.Errorf("multiple SOA records found (%d): %s", len(soas), strings.Join(names, ", "))
+	}
+}
+
 // checkUnchanged returns nil if the on-disk file still matches the
 // captured snapshot. If the file has been modified (mtime or size
 // differs), returns an error — the caller should refuse the UPDATE
@@ -299,10 +345,24 @@ func bumpSerial(rrs []dns.RR, now time.Time) error {
 	// Older or unparseable: jump to today*10000+1. Migration path for
 	// legacy YYYYMMDDNN serials lives here.
 	candidate := uint32(parseUint(today)*serialCounterMul + 1)
+
+	// H5 — explicit MaxUint32 guard. Plain `>` comparison is correct in
+	// practice (we'd never wrap during the zone's lifetime: 10000
+	// bumps/day × 365 days × ~117 years = ~427M, well under 2^32). The
+	// real failure mode we must prevent is wrap-to-0: if soa.Serial
+	// somehow reached MaxUint32 (hand-edit, fuzz, or a future code path
+	// we haven't written), `soa.Serial++` would wrap to 0, and
+	// downstream secondaries per RFC 1982 treat 0-after-MaxUint32 as
+	// "older" — they refuse to AXFR, and the zone goes dark. Loud
+	// refusal forces the operator to manually reset the serial,
+	// instead of silently bricking the zone.
 	if candidate <= soa.Serial {
-		// Defensive: don't regress. If something has somehow
-		// provisioned a serial >= today's new-format candidate (e.g.,
-		// far-future serial from a hand-edit), just +1 to advance.
+		if soa.Serial == math.MaxUint32 {
+			return fmt.Errorf("SOA serial at uint32 max (%d) — refusing to wrap to 0; operator must reset zone serial manually (see RFC 1982 §3.2)", soa.Serial)
+		}
+		// Defensive monotonic advance for the unusual "current serial
+		// is already > today's new-format minimum" case (e.g., a
+		// hand-edit set it to a far-future value).
 		soa.Serial++
 		return nil
 	}