feat(page): add typed keyset pagination for Bob (#15)

Add transport-neutral keyset pagination through native Bob query mods.
Preserve supported cursor value types in a compact versioned format, with
bidirectional pages, explicit row keys, and a separate opt-in count.
Include SQLite tests, PostgreSQL service CI, and the cursor-format ADR.

Closes #6

page/example_test.go

@@ -0,0 +1,69 @@
+package page_test
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/stephenafamo/bob"
+	"github.com/stephenafamo/bob/dialect/sqlite"
+	sd "github.com/stephenafamo/bob/dialect/sqlite/dialect"
+	"github.com/stephenafamo/bob/dialect/sqlite/sm"
+	"github.com/stephenafamo/bob/expr"
+	"github.com/stephenafamo/scan"
+	"go.auvem.com/corestack/page"
+)
+
+func Example() {
+	ctx := context.Background()
+	db, err := bob.Open("sqlite3", ":memory:")
+	if err != nil {
+		panic(err)
+	}
+	defer db.Close()
+	db.SetMaxOpenConns(1)
+	if _, err := db.ExecContext(ctx, "CREATE TABLE orders (id BIGINT PRIMARY KEY, priority BIGINT NOT NULL, tenant BIGINT NOT NULL)"); err != nil {
+		panic(err)
+	}
+	if _, err := db.ExecContext(ctx, "INSERT INTO orders VALUES (1,10,7),(2,10,7),(3,5,7),(4,20,8)"); err != nil {
+		panic(err)
+	}
+	type Order struct {
+		ID       int64
+		Priority int64
+	}
+	sort := page.Sort{ID: "orders/priority/v1", Columns: []page.Column{
+		{Expression: expr.Quote("priority"), Direction: page.Desc},
+		{Expression: expr.Quote("id"), Direction: page.Asc, Unique: true},
+	}}
+	// With Bob-generated models, use their column references for these expressions.
+	// The key must independently match their order and actual stored value types.
+	key := func(o Order) []any { return []any{o.Priority, o.ID} }
+	p, err := page.Prepare(sort, key, page.Request{Size: 2}, page.Limits{Default: 2, Maximum: 100})
+	if err != nil {
+		panic(err)
+	}
+	q := sqlite.Select(sm.Columns("id", "priority"), sm.From("orders"), page.Mod[*sd.SelectQuery](p))
+	q.Apply(sm.Where(sqlite.Quote("tenant").EQ(sqlite.Arg(int64(7)))))
+	rows, err := bob.All(ctx, db, q, scan.StructMapper[Order]())
+	if err != nil {
+		panic(err)
+	}
+	result, err := p.Page(rows)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(result.Items, result.HasNext, result.HasPrevious)
+	// Continue with Forward + result.EndCursor, or Backward + result.StartCursor.
+	// A mismatched key returns no partial Page, including for an overflow row.
+	bad, err := page.Prepare(sort, func(o Order) []any { return []any{o.ID} }, page.Request{}, page.Limits{Default: 2, Maximum: 100})
+	if err != nil {
+		panic(err)
+	}
+	_, err = bad.Page(rows)
+	var keyError *page.KeyError
+	fmt.Println(errors.As(err, &keyError), keyError.Expected, keyError.Actual)
+	// Output:
+	// [{1 10} {2 10}] true false
+	// true 2 1
+}