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/manifest.go

@@ -0,0 +1,67 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+const (
+	manifestFileName  = "campaign.json"
+	recordsFileName   = "runs.jsonl"
+	seedPlaceholder   = "{seed}"
+	devicePlaceholder = "{device}"
+)
+
+// manifest is campaign.json: what the campaign INTENDED to run, written before
+// the first run so a host that dropped runs shows up as missing seeds rather
+// than as a smaller sample.
+type manifest struct {
+	Arm               string    `json:"arm"`
+	Generator         string    `json:"generator"`
+	Platform          string    `json:"platform"`
+	SpecPath          string    `json:"spec_path"`
+	BundleID          string    `json:"bundle_id"`
+	MaxSteps          int       `json:"max_steps"`
+	DurationMillis    int64     `json:"duration_millis"`
+	Seeds             []int64   `json:"seeds"`
+	Devices           []string  `json:"devices"`
+	Host              string    `json:"host"`
+	SanderlingPath    string    `json:"sanderling_path"`
+	SanderlingVersion string    `json:"sanderling_version"`
+	StartedAt         time.Time `json:"started_at"`
+	ArgumentTemplate  []string  `json:"argument_template"`
+}
+
+func buildManifest(configuration config, host, binaryPath, version string, startedAt time.Time) manifest {
+	devices := configuration.devices
+	if devices == nil {
+		devices = []string{}
+	}
+	return manifest{
+		Arm:               configuration.arm,
+		Generator:         configuration.generator,
+		Platform:          configuration.platform,
+		SpecPath:          configuration.specPath,
+		BundleID:          configuration.bundleID,
+		MaxSteps:          configuration.maxSteps,
+		DurationMillis:    configuration.duration.Milliseconds(),
+		Seeds:             configuration.seeds,
+		Devices:           devices,
+		Host:              host,
+		SanderlingPath:    binaryPath,
+		SanderlingVersion: version,
+		StartedAt:         startedAt,
+		ArgumentTemplate:  runArguments(configuration, seedPlaceholder, devicePlaceholder),
+	}
+}
+
+func writeManifest(directory string, value manifest) error {
+	body, err := json.MarshalIndent(value, "", "  ")
+	if err != nil {
+		return fmt.Errorf("marshal manifest: %w", err)
+	}
+	return os.WriteFile(filepath.Join(directory, manifestFileName), append(body, '\n'), 0o644)
+}