perf: font-compression improvements (#1056)

## Purpose

This PR includes some preparatory changes that are needed for an
upcoming performant CJK font feature. The changes have no impact on
render time and heap allocation for latin text. **Despite this, I think
these changes stand on their own as a better font
compression/decompression implementation.**

## Summary

- Font decompressor rewrite: Replaced the 4-slot LRU group cache with a
two-tier system — a page buffer (glyphs prewarmed before rendering
begins) and a hot-group fallback (last decompressed group retained for
non-prewarmed
  glyphs). 
- Byte-aligned compressed bitmap format: Glyph bitmaps within compressed
groups are now stored row-padded rather than tightly packed before
DEFLATE compression, improving compression ratios by making identical
pixel rows produce
identical byte patterns. Glyphs are compacted back to packed format on
demand at render time. Reduces flash size by 155 KB.
- Page prewarm system: Added `Page::collectText` and
`Page::getDominantStyle` to extract per-style glyph requirements before
rendering, and `GfxRenderer::prewarmFontCache` to pre-decompress only
the groups needed for the dominant style
   — eliminating mid-render decompression for the common case.
- UTF-8 robustness fixes: `utf8NextCodepoint` now validates continuation
bytes and returns a replacement glyph on malformed input;
`ChapterHtmlSlimParser` correctly preserves incomplete multi-byte
sequences across word-buffer flush
  boundaries rather than splitting them.

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**YES**_ Architecture and
design was done by me, refined a bit by Claude. Code mostly by Claude,
but not entirely.

src/activities/reader/EpubReaderActivity.cpp

@@ -2,11 +2,13 @@
 
 #include <Epub/Page.h>
 #include <Epub/blocks/TextBlock.h>
+#include <FontCacheManager.h>
 #include <FsHelpers.h>
 #include <GfxRenderer.h>
 #include <HalStorage.h>
 #include <I18n.h>
 #include <Logging.h>
+#include <esp_system.h>
 
 #include "CrossPointSettings.h"
 #include "CrossPointState.h"
@@ -632,7 +634,6 @@ void EpubReaderActivity::render(RenderLock&& lock) {
     const auto start = millis();
     renderContents(std::move(p), orientedMarginTop, orientedMarginRight, orientedMarginBottom, orientedMarginLeft);
     LOG_DBG("ERS", "Rendered page in %dms", millis() - start);
-    renderer.clearFontCache();
   }
   saveProgress(currentSpineIndex, section->currentPage, section->pageCount);
 
@@ -662,11 +663,30 @@ void EpubReaderActivity::saveProgress(int spineIndex, int currentPage, int pageC
 void EpubReaderActivity::renderContents(std::unique_ptr<Page> page, const int orientedMarginTop,
                                         const int orientedMarginRight, const int orientedMarginBottom,
                                         const int orientedMarginLeft) {
+  const auto t0 = millis();
+  auto* fcm = renderer.getFontCacheManager();
+  fcm->resetStats();
+
+  // Font prewarm: scan pass accumulates text, then prewarm, then real render
+  const uint32_t heapBefore = esp_get_free_heap_size();
+  auto scope = fcm->createPrewarmScope();
+  page->render(renderer, SETTINGS.getReaderFontId(), orientedMarginLeft, orientedMarginTop);  // scan pass
+  scope.endScanAndPrewarm();
+  const uint32_t heapAfter = esp_get_free_heap_size();
+  fcm->logStats("prewarm");
+  const auto tPrewarm = millis();
+
+  LOG_DBG("ERS", "Heap: before=%lu after=%lu delta=%ld", heapBefore, heapAfter,
+          (int32_t)heapAfter - (int32_t)heapBefore);
+
   // Force special handling for pages with images when anti-aliasing is on
   bool imagePageWithAA = page->hasImages() && SETTINGS.textAntiAliasing;
 
   page->render(renderer, SETTINGS.getReaderFontId(), orientedMarginLeft, orientedMarginTop);
   renderStatusBar();
+  fcm->logStats("bw_render");
+  const auto tBwRender = millis();
+
   if (imagePageWithAA) {
     // Double FAST_REFRESH with selective image blanking (pablohc's technique):
     // HALF_REFRESH sets particles too firmly for the grayscale LUT to adjust.
@@ -689,9 +709,11 @@ void EpubReaderActivity::renderContents(std::unique_ptr<Page> page, const int or
   } else {
     ReaderUtils::displayWithRefreshCycle(renderer, pagesUntilFullRefresh);
   }
+  const auto tDisplay = millis();
 
   // Save bw buffer to reset buffer state after grayscale data sync
   renderer.storeBwBuffer();
+  const auto tBwStore = millis();
 
   // grayscale rendering
   // TODO: Only do this if font supports it
@@ -700,20 +722,42 @@ void EpubReaderActivity::renderContents(std::unique_ptr<Page> page, const int or
     renderer.setRenderMode(GfxRenderer::GRAYSCALE_LSB);
     page->render(renderer, SETTINGS.getReaderFontId(), orientedMarginLeft, orientedMarginTop);
     renderer.copyGrayscaleLsbBuffers();
+    const auto tGrayLsb = millis();
 
     // Render and copy to MSB buffer
     renderer.clearScreen(0x00);
     renderer.setRenderMode(GfxRenderer::GRAYSCALE_MSB);
     page->render(renderer, SETTINGS.getReaderFontId(), orientedMarginLeft, orientedMarginTop);
     renderer.copyGrayscaleMsbBuffers();
+    const auto tGrayMsb = millis();
 
     // display grayscale part
     renderer.displayGrayBuffer();
+    const auto tGrayDisplay = millis();
     renderer.setRenderMode(GfxRenderer::BW);
-  }
+    fcm->logStats("gray");
 
-  // restore the bw data
-  renderer.restoreBwBuffer();
+    // restore the bw data
+    renderer.restoreBwBuffer();
+    const auto tBwRestore = millis();
+
+    const auto tEnd = millis();
+    LOG_DBG("ERS",
+            "Page render: prewarm=%lums bw_render=%lums display=%lums bw_store=%lums "
+            "gray_lsb=%lums gray_msb=%lums gray_display=%lums bw_restore=%lums total=%lums",
+            tPrewarm - t0, tBwRender - tPrewarm, tDisplay - tBwRender, tBwStore - tDisplay, tGrayLsb - tBwStore,
+            tGrayMsb - tGrayLsb, tGrayDisplay - tGrayMsb, tBwRestore - tGrayDisplay, tEnd - t0);
+  } else {
+    // restore the bw data
+    renderer.restoreBwBuffer();
+    const auto tBwRestore = millis();
+
+    const auto tEnd = millis();
+    LOG_DBG("ERS",
+            "Page render: prewarm=%lums bw_render=%lums display=%lums bw_store=%lums bw_restore=%lums total=%lums",
+            tPrewarm - t0, tBwRender - tPrewarm, tDisplay - tBwRender, tBwStore - tDisplay, tBwRestore - tBwStore,
+            tEnd - t0);
+  }
 }
 
 void EpubReaderActivity::renderStatusBar() const {

src/activities/reader/TxtReaderActivity.cpp

@@ -1,5 +1,6 @@
 #include "TxtReaderActivity.h"
 
+#include <FontCacheManager.h>
 #include <GfxRenderer.h>
 #include <HalStorage.h>
 #include <I18n.h>
@@ -320,7 +321,6 @@ void TxtReaderActivity::render(RenderLock&&) {
 
   renderer.clearScreen();
   renderPage();
-  renderer.clearFontCache();
 
   // Save progress
   saveProgress();
@@ -365,7 +365,13 @@ void TxtReaderActivity::renderPage() {
     }
   };
 
-  // First pass: BW rendering
+  // Font prewarm: scan pass accumulates text, then prewarm, then real render
+  auto* fcm = renderer.getFontCacheManager();
+  auto scope = fcm->createPrewarmScope();
+  renderLines();  // scan pass — text accumulated, no drawing
+  scope.endScanAndPrewarm();
+
+  // BW rendering
   renderLines();
   renderStatusBar();
 
@@ -374,6 +380,7 @@ void TxtReaderActivity::renderPage() {
   if (SETTINGS.textAntiAliasing) {
     ReaderUtils::renderAntiAliased(renderer, [&renderLines]() { renderLines(); });
   }
+  // scope destructor clears font cache via FontCacheManager
 }
 
 void TxtReaderActivity::renderStatusBar() const {