docs(08): create phase plans for image display fix and pagination cache

3 plans covering QUAL-01 through QUAL-04:
- 08-01: pagination cache key + wiring into rendering pipeline
- 08-02: unified 1080x1920 max-size for all images, footnote width-only fix
- 08-03: pagination timing instrumentation

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

.planning/phases/08-pagination-quality-cache-performance/08-VALIDATION.md

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+# Phase 8: Validation Plan
+
+**Phase:** 08-pagination-quality-cache-performance
+**Created:** 2026-05-23
+
+## Verification Strategy
+
+This phase has no unit test target for EPUBTextRendering. Verification is build-only plus manual visual inspection.
+
+### Automated Verification
+
+| Check | Command | Frequency |
+|-------|---------|-----------|
+| Build succeeds | `xcodebuild build -project ReadViewDemo/ReadViewDemo.xcodeproj -scheme ReadViewDemo -destination 'platform=iOS Simulator,name=iPhone 16'` | Per task commit |
+
+### Manual Verification (Phase Gate)
+
+| Requirement | What to Check | How |
+|-------------|---------------|-----|
+| QUAL-01 | Same chapter does not re-typeset on repeat navigation | Navigate to a chapter, back, then forward again — second visit should be noticeably faster (check `[EPUB][Perf]` log for elapsed time) |
+| QUAL-02 | Images don't overflow pages | Open 宝山辽墓材料与释读, navigate to Chapter 5 (qrbodyPic images), verify no height overflow |
+| QUAL-03 | Image sizing rules are verifiable | Code review: `DTMaxImageSize` = 1080x1920, unified max-size block in `prepareHTMLElementForReaderRendering` |
+| QUAL-04 | No perf degradation | Check `[EPUB][Perf]` log output — pagination times should be comparable to pre-change baseline |
+
+### Image Display Checklist
+
+- [ ] Cover image: displays correctly, not cropped, not stretched
+- [ ] qrbodyPic images: display within page bounds, no height overflow
+- [ ] Footnote images: inline with text, same height as text characters, width-only sizing
+- [ ] Dark mode: all image types still display correctly
+- [ ] Page breaks around images: no orphan images at page tops/bottoms
+
+## Rationale
+
+Build-only verification is the appropriate approach for this phase because:
+1. No unit test target exists for EPUBTextRendering code
+2. Image sizing correctness is inherently visual — pixel-level assertions would be brittle
+3. The changes are surgical (one new code block, removals of redundant logic) and can be verified by code review + visual inspection
+4. Creating a test infrastructure is out of scope (belongs to Phase 9: 自动化回归与证据标准化)