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/.env.example

@@ -0,0 +1,8 @@
+# Copy to .env and adjust paths for your setup.
+# direnv loads these automatically (see .envrc).
+
+# Kernel source / prepared headers directory
+KERNEL_SRC=~/pixel-kernel-headers
+
+# Clang toolchain bin directory (must contain clang, ld.lld, llvm-ar, etc.)
+CLANG_DIR=~/pixel-kernel/prebuilts/clang/host/linux-x86/clang-r487747c/bin