feat(koreader): one conversion route for every feature; bookmarks get web CFIs

Progress, highlights, notes, and bookmarks entered position conversion
through three different doors: progress converted inline with an
uncached converter, annotations through the facade, bookmarks not at
all (the raw xpointer was stored verbatim, cfi_position stayed empty,
and the web drawer's goToBookmark silently no-ops on cfi-less entries).

Unify on the facade (ConvertToCanonical/ConvertFromCanonical):

- annotationEpub context resolved once per push: media item + EPUB path
  shared by every annotation instead of re-fetched per entry
- progress forward: the inline block becomes one facade call;
  non-reflowable formats pass through unchanged, and the cached
  converter stops re-parsing the book on every sync
- progress reverse: convertCFIToXPointer delegates to reverseConvertCFI,
  keeping the stored percentage in play for the fallback ladder
- bookmarks (bulk progress and /sync-bookmarks): pos0 resolves
  structural-only — bookmark text is a display label, never book text,
  so no context is supplied; webUsableCFI stores the result only for
  structural/exact epubcfi landings, discarding href/percentage results
  rather than storing dead drawer links. Also records percentage_location
  and origin_source on the legacy endpoint.
- percentages thread through: highlights/notes/bookmarks pass the device
  percentage or the derived section percentage instead of a hardcoded 0,
  so the last-resort fallback lands near the true position instead of
  the document start
- extendCFIByLength end-derivation now also fires on structural starts
  (it had silently stopped matching when the structural rung began
  landing starts with precision 'structural' rather than 'exact')

Tests: the drop-cap xpointer through the facade with empty context (the
bookmark scenario) must land structurally, not doc-start; webUsableCFI
table covers the store/discard gate.

internal/handlers/koreader_test.go

@@ -4,6 +4,8 @@ import (
 	"encoding/json"
 	"testing"
 
+	wsync "bookhoard/internal/sync"
+
 	"github.com/stretchr/testify/assert"
 )
 
@@ -50,3 +52,53 @@ func TestKOReaderProgressRequest_DeletedAnnotationsOmitted(t *testing.T) {
 	assert.Empty(t, req.Books[0].DeletedHighlights)
 	assert.Empty(t, req.Books[0].DeletedBookmarks)
 }
+
+// The web drawer navigates bookmarks by CFI, so only high-confidence
+// conversions may be stored: href/percentage/fallback results would
+// become dead links. This is the sole gate for KOReader→web bookmark
+// positions (bookmark text is a label, never book text, so the
+// conversion runs structural-only with empty context).
+func TestWebUsableCFI(t *testing.T) {
+	cases := []struct {
+		name     string
+		loc      wsync.CanonicalLocator
+		expected string
+	}{
+		{
+			name:     "structural landing stored",
+			loc:      wsync.CanonicalLocator{CFI: "epubcfi(/6/52!/4/28/2/1:0)", Precision: "structural", Percentage: 0.52},
+			expected: "epubcfi(/6/52!/4/28/2/1:0)",
+		},
+		{
+			name:     "exact text-search landing stored",
+			loc:      wsync.CanonicalLocator{CFI: "epubcfi(/6/52!/4/28/2/1:0)", Precision: "exact", Percentage: 0.52},
+			expected: "epubcfi(/6/52!/4/28/2/1:0)",
+		},
+		{
+			name:     "percentage guess discarded",
+			loc:      wsync.CanonicalLocator{CFI: "epubcfi(/6/52!/4/2/1:0)", Precision: "percentage", Percentage: 0.52},
+			expected: "",
+		},
+		{
+			name:     "fallback passthrough (raw xpointer) discarded",
+			loc:      wsync.CanonicalLocator{CFI: "/body/DocFragment[2]/body/p[3]", Precision: "fallback", Percentage: 0.52},
+			expected: "",
+		},
+		{
+			name:     "section href discarded (not a CFI)",
+			loc:      wsync.CanonicalLocator{CFI: "ch10.xhtml", Precision: "section", Percentage: 0.52},
+			expected: "",
+		},
+		{
+			name:     "structural but non-CFI value discarded",
+			loc:      wsync.CanonicalLocator{CFI: "ch10.xhtml#h1", Precision: "structural", Percentage: 0.52},
+			expected: "",
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.Equal(t, tc.expected, webUsableCFI(tc.loc))
+		})
+	}
+}