feat: M1 determinism fixes for gossip

Refs #1

- F1: canonical fingerprints (git identity by remote URL, log by
  basename, obsidian by relative vault path) so identical facts get
  identical ids across machines
- F2: node_id (persisted in settings) + Hybrid Logical Clock in all
  observation "when" columns; removed time.Now() fact-time fallbacks
- F3: stable TF-IDF tie-break sort (score desc, term asc)
- F4: observations carry (node_id, hcl) locator; dedup ordered by hcl

internal/db/db.go

@@ -1,7 +1,9 @@
 package db
 
 import (
+	"crypto/rand"
 	"database/sql"
+	"encoding/hex"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -10,13 +12,16 @@ import (
 	"sync"
 	"time"
 
+	"github.com/david/knox/internal/hlc"
 	_ "modernc.org/sqlite"
 )
 
 const goldenMinConfidence = 0.7
 
 type KnoxDB struct {
-	db *sql.DB
+	db     *sql.DB
+	nodeID string
+	clock  *hlc.Clock
 }
 
 type Observation struct {
@@ -101,6 +106,9 @@ func Open(path string) (*KnoxDB, error) {
 	migrations := []string{
 		"ALTER TABLE threads ADD COLUMN provenance TEXT DEFAULT '{}'",
 		"ALTER TABLE entries ADD COLUMN created_at TEXT DEFAULT ''",
+		"ALTER TABLE observations ADD COLUMN node_id TEXT NOT NULL DEFAULT ''",
+		"ALTER TABLE observations ADD COLUMN hcl INTEGER",
+		"ALTER TABLE observations ADD COLUMN received_at TEXT",
 	}
 	for _, m := range migrations {
 		if _, err := db.Exec(m); err != nil && !strings.Contains(err.Error(), "duplicate column") {
@@ -108,9 +116,54 @@ func Open(path string) (*KnoxDB, error) {
 		}
 	}
 
-	return &KnoxDB{db: db}, nil
+	kdb := &KnoxDB{db: db, clock: hlc.New()}
+
+	// Node identity: generated once, persisted in settings. Stable across
+	// restarts so gossip cursors and locator IDs survive.
+	nodeID, err := kdb.loadNodeID()
+	if err != nil {
+		return nil, err
+	}
+	kdb.nodeID = nodeID
+
+	// Backfill: rows added before node_id existed belong to this node.
+	if _, err := db.Exec("UPDATE observations SET node_id=? WHERE node_id=''", nodeID); err != nil {
+		return nil, fmt.Errorf("backfill node_id: %w", err)
+	}
+
+	return kdb, nil
 }
 
+const nodeIDKey = "node_id"
+
+// loadNodeID reads the persisted node identity, creating one if absent.
+func (k *KnoxDB) loadNodeID() (string, error) {
+	var id string
+	err := k.db.QueryRow(`SELECT value FROM settings WHERE key=?`, nodeIDKey).Scan(&id)
+	if err == nil && id != "" {
+		return id, nil
+	}
+	if err != nil && err != sql.ErrNoRows {
+		return "", fmt.Errorf("read node_id: %w", err)
+	}
+
+	buf := make([]byte, 16)
+	if _, err := rand.Read(buf); err != nil {
+		return "", fmt.Errorf("generate node_id: %w", err)
+	}
+	id = hex.EncodeToString(buf)
+	if _, err := k.db.Exec(
+		`INSERT INTO settings (key, value) VALUES (?, ?) ON CONFLICT(key) DO UPDATE SET value=?`,
+		nodeIDKey, id, id,
+	); err != nil {
+		return "", fmt.Errorf("persist node_id: %w", err)
+	}
+	return id, nil
+}
+
+// NodeID returns this node's stable identity.
+func (k *KnoxDB) NodeID() string { return k.nodeID }
+
 func (k *KnoxDB) Close() error { return k.db.Close() }
 
 // RecordObservation appends an immutable observation and updates the entries cache.
@@ -123,7 +176,8 @@ func (k *KnoxDB) RecordObservation(o ObservationRecord) (obsID int64, isNew bool
 	}
 	defer tx.Rollback()
 
-	now := time.Now().UTC().Format(time.RFC3339)
+	hclVal, hclWall := k.clock.Now()
+	now := hclWall.Format(time.RFC3339)
 
 	// Dedup: compare against the latest observation of the same fingerprint.
 	// For content sources (LineEnd > 0) the extent/summary IS the change signal —
@@ -136,7 +190,7 @@ func (k *KnoxDB) RecordObservation(o ObservationRecord) (obsID int64, isNew bool
 	err = tx.QueryRow(
 		`SELECT id, COALESCE(title,''), COALESCE(summary,''), COALESCE(project,''),
 		        COALESCE(created_at,''), COALESCE(line_start,0), COALESCE(line_end,0)
-		 FROM observations WHERE fingerprint=? ORDER BY id DESC LIMIT 1`, o.Fingerprint,
+		 FROM observations WHERE fingerprint=? ORDER BY hcl DESC, id DESC LIMIT 1`, o.Fingerprint,
 	).Scan(&prevID, &pTitle, &pSummary, &pProject, &pCreated, &pStart, &pEnd)
 	switch err {
 	case nil:
@@ -154,10 +208,12 @@ func (k *KnoxDB) RecordObservation(o ObservationRecord) (obsID int64, isNew bool
 	res, err := tx.Exec(
 		`INSERT INTO observations
 		 (fingerprint, source_id, source_path, project, content_type, title, summary,
-		  collected_at, created_at, line_start, line_end, confidence, ingester_version, trigger, provenance)
-		 VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+		  collected_at, created_at, line_start, line_end, confidence, ingester_version, trigger, provenance,
+		  node_id, hcl, received_at)
+		 VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
 		o.Fingerprint, o.SourceID, o.SourcePath, o.Project, o.ContentType, o.Title, o.Summary,
 		now, o.CreatedAt, o.LineStart, o.LineEnd, o.Confidence, o.IngesterVersion, o.Trigger, o.Provenance,
+		k.nodeID, hclVal, time.Now().UTC().Format(time.RFC3339),
 	)
 	if err != nil {
 		return 0, false, fmt.Errorf("insert observation: %w", err)