feat: add PromoteToInteractive and DemoteToDocument for mid-session page transfer

Allow transferring ownership of a Playwright page between Document and
InteractiveBrowser modes without tearing down the browser. This enables
handing a live page to a human (e.g. for captcha solving) and resuming
scraping on the same page afterward.

Closes #76

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

promote.go

@@ -0,0 +1,65 @@
+package extractor
+
+import "errors"
+
+// ErrNotPromotable is returned when a Document cannot be promoted to an InteractiveBrowser.
+// This happens when the Document is not backed by a Playwright page (e.g. a mock or custom implementation).
+var ErrNotPromotable = errors.New("document is not promotable to InteractiveBrowser")
+
+// ErrNotDemotable is returned when an InteractiveBrowser cannot be demoted to a Document.
+// This happens when the InteractiveBrowser is not backed by a Playwright page.
+var ErrNotDemotable = errors.New("interactive browser is not demotable to Document")
+
+// ErrAlreadyDetached is returned when attempting to promote or demote an object that has
+// already been transferred. Each Document or InteractiveBrowser can only be promoted/demoted once.
+var ErrAlreadyDetached = errors.New("already detached")
+
+// PromoteToInteractive transfers ownership of the underlying Playwright page from a Document
+// to a new InteractiveBrowser. After promotion, the Document's Close method becomes a no-op
+// (the page is now owned by the returned InteractiveBrowser).
+//
+// The caller must keep the original Browser alive while the promoted InteractiveBrowser is in use,
+// since the Browser still owns the Playwright process and browser instance.
+//
+// Returns ErrNotPromotable if the Document is not backed by a Playwright page,
+// or ErrAlreadyDetached if the Document was already promoted.
+func PromoteToInteractive(doc Document) (InteractiveBrowser, error) {
+	d, ok := doc.(*document)
+	if !ok {
+		return nil, ErrNotPromotable
+	}
+
+	if d.detached {
+		return nil, ErrAlreadyDetached
+	}
+
+	d.detached = true
+
+	return &interactiveBrowser{
+		pw:      d.pw,
+		browser: d.browser,
+		ctx:     d.page.Context(),
+		page:    d.page,
+	}, nil
+}
+
+// DemoteToDocument transfers ownership of the underlying Playwright page from an
+// InteractiveBrowser back to a new Document. After demotion, the InteractiveBrowser's
+// Close method becomes a no-op (the page is now owned by the returned Document).
+//
+// Returns ErrNotDemotable if the InteractiveBrowser is not backed by a Playwright page,
+// or ErrAlreadyDetached if the InteractiveBrowser was already demoted.
+func DemoteToDocument(ib InteractiveBrowser) (Document, error) {
+	b, ok := ib.(*interactiveBrowser)
+	if !ok {
+		return nil, ErrNotDemotable
+	}
+
+	if b.detached {
+		return nil, ErrAlreadyDetached
+	}
+
+	b.detached = true
+
+	return newDocument(b.pw, b.browser, b.page)
+}