perf(reader): M31 字体路径记忆化（开书字体查找 35 目录 walk → 1 次 stat）+ 「· 读」徽标即时刷新

字体：find_font_rec/resolve_font 从 reader.c 抽到 app/font_resolve.c 并加进程内记忆化。
只缓存成功路径；命中后仍做一次 access() 校验（字库被删即重扫）；没找到不缓存，
用户之后拷入的字库下次开书即可找到。顶栏「刷新」额外调 font_cache_invalidate()，
给「新拷入字库、旧字库仍在」一条显式失效路径。宿主实测：35 目录树 walk 0.026~0.54ms
（=300+ 次系统调用）vs 命中 0.001ms（1 次 access）；开书路径每次省一次完整 walk。

徽标：副题格式化抽成 shelf_fmt_sub()（扫描与「刚读完置顶」共用一份实现），
promote_just_read() 改 ts 后立即重算这本的 sub/sub_folder，「· 读」不再等下次全量刷新。

回归：test_reader_core 6846、shelf_group_smoke 63（+8）、font_resolve_smoke 14 全绿；
build_reader.sh 0 warning；扫描缓存/进度语义未改。app.json 0.1.3/code4 → 0.1.4/code5。

patch_v2/reader_app/build_reader.sh

@@ -6,6 +6,9 @@
 # 产物：
 #   reader                          mipsel 静态 ELF（链 meow_ui.a + freetype + miniz + libmobi + zlib）
 #   test/test_reader_core_host      宿主 core 单测（enc/layout/txt）
+#   test/shelf_group_smoke_host     宿主书架分组单测（M27/M31 副题）
+#   test/font_resolve_smoke_host    宿主字体路径记忆化单测（M31）
+#   test/font_resolve_bench_host    宿主字体 walk/命中耗时对比（M31，只编不跑）
 #   test/loaders_smoke_host         宿主四格式 loader 单测
 #   test/rdft_smoke_mipsel          qemu-mipsel FreeType 冒烟
 #   test/book_smoke_mipsel          qemu-mipsel TXT 书桥冒烟
@@ -116,6 +119,11 @@ echo "==> host shelf group test (M27)"
 $HOST_CC -O2 -Wall -Wextra -Iapp -I../sdk test/shelf_group_smoke.c app/shelf_model.c app/progress.c app/reader_conf.c -o test/shelf_group_smoke_host
 ./test/shelf_group_smoke_host
 
+echo "==> host font resolve test (M31)"
+$HOST_CC -O2 -Wall -Wextra -Iapp test/font_resolve_smoke.c app/font_resolve.c -o test/font_resolve_smoke_host
+./test/font_resolve_smoke_host
+$HOST_CC -O2 -Wall -Wextra -Iapp test/font_resolve_bench.c app/font_resolve.c -o test/font_resolve_bench_host
+
 echo "==> host shelf scan bench (M27，只编不跑；用法 ./test/shelf_scan_bench_host <Books目录> [progress|noop] [轮数])"
 $HOST_CC -O2 -Wall -Wextra -Iapp -I../sdk test/shelf_scan_bench.c app/shelf_model.c app/progress.c -o test/shelf_scan_bench_host
 
@@ -161,7 +169,7 @@ run_qemu 150 "$QEMU_BIN" ./test/formats_smoke_mipsel "${FONT:-$ROOT/font/src/MiS
 echo "==> reader binary"
 CORE_L="core/rdenc.c core/rdlayout.c core/rdtxt.c core/seg.c core/rdchapter.c core/rdxml.c core/rdhtml.c core/rdsb.c core/rdzip.c core/rdsrc.c core/rdepub.c core/rdfb2.c core/rdmobi.c core/rdcbz.c"
 ENGINE_L="engine/rdft.c"
-APP_L="app.c app/shelf.c app/shelf_model.c app/reader.c app/book.c app/progress.c app/settings.c app/reader_conf.c app/reader_ipc.c"
+APP_L="app.c app/shelf.c app/shelf_model.c app/font_resolve.c app/reader.c app/book.c app/progress.c app/settings.c app/reader_conf.c app/reader_ipc.c"
 $CC -Os -static -Wall -Wextra -I../sdk -I../meow_launcherd -I. -Icore -Iapp -Iengine $FT_INCS $COREINC \
   $CORE_L $APP_L $ENGINE_L "$OBJ"/mz_*.o "$OBJ"/lm_*.o "$OBJ"/rdimg.o \
   ../sdk/meow_ui.a "$SYSROOT/lib/libz.a" "$FT_BUILD/libfreetype.a" -lpthread -o reader
@@ -173,7 +181,7 @@ echo "  运行(qemu): qemu-mipsel ./reader"
 # ---------- 8) 真机耗时基准（不进发行包：与 reader 同源，只把 app.c 的 main 换成 bench main） ----------
 # 用法（设备）：MEOW_FONT_* 等 env 与 App 一致时执行 ./shelf_bench_ondev_mipsel [书...]
 echo "==> ondev shelf bench (not shipped)"
-APP_BENCH_L="app/shelf.c app/shelf_model.c app/reader.c app/book.c app/progress.c app/settings.c app/reader_conf.c app/reader_ipc.c"
+APP_BENCH_L="app/shelf.c app/shelf_model.c app/font_resolve.c app/reader.c app/book.c app/progress.c app/settings.c app/reader_conf.c app/reader_ipc.c"
 $CC -Os -static -Wall -Wextra -I../sdk -I../meow_launcherd -I. -Icore -Iapp -Iengine $FT_INCS $COREINC \
   test/shelf_bench_ondev.c $CORE_L $APP_BENCH_L $ENGINE_L "$OBJ"/mz_*.o "$OBJ"/lm_*.o "$OBJ"/rdimg.o \
   ../sdk/meow_ui.a "$SYSROOT/lib/libz.a" "$FT_BUILD/libfreetype.a" -lpthread -o test/shelf_bench_ondev_mipsel