feat(campaign): sweep seeds for one experiment cell

campaign.json lists the seeds a sweep intended to run and is written before
the first run, so a host that dropped runs shows up as missing seeds rather
than as a smaller sample. Seed 0 is rejected: sanderling test reads it as
"derive a seed from the clock", which is why conformance/gates.sh controls
nothing today.

Each run contributes one runs.jsonl line carrying steps to first violation by
origin step, the step that armed the failed obligation, so the survival
analysis never reopens a trace.

Claude-Session: https://claude.ai/code/session_01A5KmftdEJ49A9z5mF5ESrX

cmd/internal-tools/campaign/seeds.go

@@ -0,0 +1,79 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+// parseSeeds expands a seed specification such as "1-10,20,30-32" into the
+// explicit seed list a campaign intends to run.
+func parseSeeds(specification string) ([]int64, error) {
+	trimmed := strings.TrimSpace(specification)
+	if trimmed == "" {
+		return nil, fmt.Errorf("empty seed spec")
+	}
+	var seeds []int64
+	seen := map[int64]bool{}
+	for _, part := range strings.Split(trimmed, ",") {
+		part = strings.TrimSpace(part)
+		if part == "" {
+			return nil, fmt.Errorf("empty seed in %q", specification)
+		}
+		expanded, err := expandSeedPart(part)
+		if err != nil {
+			return nil, err
+		}
+		for _, seed := range expanded {
+			if seen[seed] {
+				return nil, fmt.Errorf("duplicate seed %d in %q", seed, specification)
+			}
+			seen[seed] = true
+			seeds = append(seeds, seed)
+		}
+	}
+	return seeds, nil
+}
+
+func expandSeedPart(part string) ([]int64, error) {
+	start, end, isRange := strings.Cut(part, "-")
+	if !isRange {
+		seed, err := parseSeed(part)
+		if err != nil {
+			return nil, err
+		}
+		return []int64{seed}, nil
+	}
+	first, err := parseSeed(strings.TrimSpace(start))
+	if err != nil {
+		return nil, fmt.Errorf("seed range %q: %w", part, err)
+	}
+	last, err := parseSeed(strings.TrimSpace(end))
+	if err != nil {
+		return nil, fmt.Errorf("seed range %q: %w", part, err)
+	}
+	if first > last {
+		return nil, fmt.Errorf("seed range %q: start %d is above end %d", part, first, last)
+	}
+	seeds := make([]int64, 0, last-first+1)
+	for seed := first; seed <= last; seed++ {
+		seeds = append(seeds, seed)
+	}
+	return seeds, nil
+}
+
+func parseSeed(text string) (int64, error) {
+	seed, err := strconv.ParseInt(text, 10, 64)
+	if err != nil {
+		return 0, fmt.Errorf("invalid seed %q: want a positive integer", text)
+	}
+	if seed == 0 {
+		// `sanderling test` reads --seed 0 as "derive a seed from the clock",
+		// so a campaign listing seed 0 records a run nobody can reproduce.
+		return 0, fmt.Errorf("seed 0 is not reproducible: sanderling test derives a random seed when --seed is 0, so list explicit non-zero seeds")
+	}
+	if seed < 0 {
+		return 0, fmt.Errorf("invalid seed %d: want a positive integer", seed)
+	}
+	return seed, nil
+}