feat(sync): server-side position authority — verify/heal progress anchors

Progress submissions now carry (percentage, context_text, epubcfi) and
the server becomes the position authority:

- VerifyProgressAnchor resolves the submitted standard CFI against the
  book's own XHTML, extracts the text at the anchor, and cross-checks it
  with the submitted context_text. A mismatch heals the anchor by text
  search (percentage disambiguates repeats) instead of storing a bad
  position.
- The anchor's block element is derived as a cssSelector plus a block-
  relative character offset, and served on progress GET alongside the
  anchor document's href — readium-native handles that let clients
  re-open a book without parsing CFIs themselves.
- context_text-only submissions (no CFI — the dumb-client tier) are
  anchored structurally from the context text.

Motivation: cross-client progress sync (web foliate CFIs, KOReader CRE
xpointers, readium-native apps) previously trusted each client's own
locator math; the app's EPUB restore drifted ±pages because readium's
paginator does not lay out far-from-viewport columns and the foliate-
ported CFI walk ran against readium's mutated WebView DOM. Server-side
verification heals both classes at ingest.

internal/sync/cfi_verify_manual_test.go

@@ -0,0 +1,22 @@
+package sync
+
+import (
+	"os"
+	"testing"
+)
+
+func TestManualRealBookAnchor(t *testing.T) {
+	path := "/home/nymusicman/Code/bookhoard/uploads/Ebooks/George Orwell/1984 (1269)/1984 - George Orwell.epub"
+	if _, err := os.Stat(path); err != nil {
+		t.Skip("real book not present on this machine")
+	}
+	cfi := "epubcfi(/6/2!/4[x1984]/8[_idContainer003]/62/1:456)"
+	ctx := "was at war with one of these Powers it was generally at peace with the other"
+	finalCFI, sel, href, charOff, healedPct, healed, err := VerifyProgressAnchor(path, cfi, ctx, 0.0415)
+	if err != nil {
+		t.Fatalf("VerifyProgressAnchor error: %v", err)
+	}
+	t.Logf("final=%q\n healed=%v healedPct=%v\n selector=%q href=%q charOff=%v", finalCFI, healed, healedPct, sel, href, charOff)
+	sel2, err2 := ProgressCSSSelector(path, cfi)
+	t.Logf("ProgressCSSSelector: %q err=%v", sel2, err2)
+}