refactor: overhaul kmod build system, fix kernel module bugs

Build system:
- Replace hardcoded paths in Makefile with env vars (KERNEL_SRC, CLANG_DIR)
- Add .env.example and .envrc for direnv-based config
- Simplify build-zip.sh to delegate to make instead of duplicating build command
- Rewrite BUILDING.md: 5-step happy path with direnv, standalone prep as appendix
- Remove redundant quick-reference script and step 7 (manual module.lds hack)

Kernel module (vpnhide_kmod.c):
- Fix fib_route_seq_show hook: save seq_file pointer and buffer position in entry
  handler instead of reading regs->regs[0] in return handler (which holds the
  return value on arm64, not the original argument). Rewrite buffer scanning as
  clean forward iteration with memmove compaction.
- Remove dead SIOCGIFCONF case from dev_ioctl hook (confirmed in kernel source:
  SIOCGIFCONF goes through sock_ioctl -> dev_ifconf, not dev_ioctl on GKI 6.1)
- Fix header comment: remove false tcp4_seq_show claim, correct rtnl symbol name

Test app:
- Auto-run checks on launch (LaunchedEffect) for easier adb-driven testing

kmod/build-zip.sh

@@ -3,21 +3,10 @@ set -euo pipefail
 
 cd "$(dirname "$0")"
 
-# Build the kernel module if .ko is not fresh
-KSRC="${KSRC:-$HOME/tmp_zfs/kernel_build/kernel-source}"
-CLANG="${CLANG:-$HOME/Android/Sdk/ndk/29.0.14033849/toolchains/llvm/prebuilt/linux-x86_64/bin}"
-
+# Build the kernel module (env vars loaded by direnv from .env)
 if [ ! -f vpnhide_kmod.ko ] || [ vpnhide_kmod.c -nt vpnhide_kmod.ko ]; then
     echo "Building kernel module..."
-    make -C "$KSRC" M=$(pwd) \
-        ARCH=arm64 LLVM=1 LLVM_IAS=1 \
-        CC=$CLANG/clang LD=$CLANG/ld.lld \
-        AR=$CLANG/llvm-ar NM=$CLANG/llvm-nm \
-        OBJCOPY=$CLANG/llvm-objcopy \
-        OBJDUMP=$CLANG/llvm-objdump \
-        STRIP=$CLANG/llvm-strip \
-        CROSS_COMPILE=aarch64-linux-gnu- \
-        modules
+    make
 fi
 
 cp vpnhide_kmod.ko module/vpnhide_kmod.ko