Add FreeInkBook EPUB engine migration guide

Document migration plan from CrossPoint's lib/Epub to FreeInkBook, a memory-efficient clean-room EPUB engine. Covers advantages (O(paragraph+page) memory via SAX parsing, arena allocation, host-testable, full UAX#14 typography, runtime TTF/OTF support), 9-step migration strategy, ESP32-C3 constraints, and integration approach preserving existing UI layer while replacing parsing/layout stack.

platformio.ini

@@ -26,9 +26,6 @@ build_flags =
   -DEINK_DISPLAY_SINGLE_BUFFER_MODE=1
   -DDISABLE_FS_H_WARNING=1
   -DDESTRUCTOR_CLOSES_FILE=1
-# https://libexpat.github.io/doc/api/latest/#XML_GE
-  -DXML_GE=0
-  -DXML_CONTEXT_BYTES=1024
   -std=gnu++2a
 # Enable UTF-8 long file names in SdFat
   -DUSE_UTF8_LONG_NAMES=1
@@ -37,6 +34,10 @@ build_flags =
   -DPNG_MAX_BUFFERED_PIXELS=16416
   -DFREEINK_DEVICE_X4=1
   -DFREEINK_DEVICE_X3=1
+# FreeInkBook memory profile for PSRAM-less ESP32-C3 (see BookProfile.h).
+# The firmware's only expat is FreeInkBook's vendored copy (configured by its
+# expat_config.h); consumers include <epub/Expat.h>.
+  -DFREEINK_BOOK_SMALL=1
   -Wno-bidi-chars
   -Wl,--wrap=panic_print_backtrace,--wrap=panic_abort,--wrap=bootloader_common_check_efuse_blk_validity
   -fno-exceptions
@@ -67,6 +68,7 @@ lib_deps =
   PowerManager=symlink://freeink-sdk/libs/hardware/PowerManager
   FreeInkUI=symlink://freeink-sdk/libs/ui/FreeInkUI
   Icons=symlink://freeink-sdk/libs/assets/Icons
+  FreeInkBook=symlink://freeink-sdk/libs/book/FreeInkBook
   bblanchon/ArduinoJson @ 7.4.2
   ricmoo/QRCode @ 0.0.1
   bitbank2/PNGdec @ 1.1.6