docs: document build architecture in INTERFACES.sh

Add a 'Build Architecture' section explaining the substrate
approach (Ventoy modsrc as the GRUB binary), the runtime bridge
via the hiperiso_boot GRUB script function, the two hardcoded
layout constraints (FAT label 'VTOYEFI', /ventoy/ventoy.cpio at
the ESP root), and the modsrc sed-patch strategy.

Without this, a future contributor reading the build script will
not understand why BOOTX64.EFI has ventoy_* symbols and the source
tree has unused hiperiso_*.c files; they will likely try another
incomplete rebrand.

INTERFACES.sh

@@ -167,3 +167,34 @@ HIPERISO_JSON="${DATA_MOUNT}/hiperiso/hiperiso.json"
 # Usage: hiperiso-log trace <trace.bin> [--format json|csv|text]
 # Usage: hiperiso-log serial <serial.log> [--stages]
 # Output: writes report.json, report.txt, and analysis.meta into <log_dir>
+
+# ── Build Architecture ─────────────────────────────────────────────────────
+# The GRUB binary shipped as $EFI_PAYLOAD_DIR/../BOOTX64.EFI is built from
+# Ventoy 2.04 + vendor/grub2-modsrc.tar.xz (Ventoy's GRUB core patches).
+# This is the "substrate" approach: rather than rename Ventoy symbols to
+# hiperiso_*, we keep the Ventoy module intact and bridge to a hiperiso
+# runtime via two channels:
+#
+#   1. Runtime cmdline:  the GRUB script function `hiperiso_boot` in
+#      $GRUB_CFG constructs a `hiperiso_iso=...` cmdline and runs
+#      `linux` + `initrd` + `boot` against the host kernel + QEMU
+#      initramfs under $EFI_PAYLOAD_DIR/.
+#
+#   2. Layout:  the FAT16 volume label MUST be "VTOYEFI" and the
+#      /ventoy/ventoy.cpio file MUST exist at the ESP root — both are
+#      hardcoded checks in the modsrc's ventoy_check_official_device().
+#      The label and file are produced by scripts/package_release.sh.
+#
+# Modsrc patches: scripts/build_grub2_204.sh applies a single sed
+# pass to the unpacked modsrc tree to (1) accept a 64MB ESP (modsrc
+# hardcodes 32MB / 65536 sectors) and (2) tolerate the missing 0x22
+# GPT-MBR marker byte. (The 0x22 marker is written by the installer
+# in HiperisoWorker.sh — not by the build.) Anything beyond these
+# patches is upstream Ventoy territory; do not modify modsrc without
+# bumping the version check in scripts/build_all.sh.
+#
+# The hiperiso-specific C sources in src/grub2/hiperiso_*.c are kept
+# in-tree as historical reference but are not compiled or shipped.
+# They document an earlier (incomplete) rebrand attempt; their
+# unmodified use would break moddep.lst generation.
+