build: introduce mk/boards.mk board target table, T= selects board

T= now names a specific board (qemu, pc, pi4, pi5, milkv) rather than
an arch alias. mk/boards.mk is the single table: sets ARCH, BOARD, and
BOARD_NAME. Each Makefile applies board-specific compiler flags after its
arch block — pi5 gets Cortex-A76 tuning, pi4 gets Cortex-A72, qemu uses
portable x86-64 baseline instead of -march=native. Zynq-7020 is stubbed
in comments for future 32-bit ARM support. ARCH= override still works
for raw arch selection without a named board.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Makefile

@@ -4,9 +4,11 @@
 #
 # Quick Start:
 #   make                          - Build for host arch (fastest profile, always)
-#   make T=raspi                  - Build for Raspberry Pi 4  (T= is a shorthand for ARCH=)
-#   make T=amd64                  - Build for x86_64
-#   make T=riscv64                - Build for RISC-V 64
+#   make T=qemu                   - Build portable x86-64 for QEMU
+#   make T=pc                     - Build for x86-64 PC (native tuning)
+#   make T=pi4                    - Cross-compile for Raspberry Pi 4 (Cortex-A72)
+#   make T=pi5                    - Cross-compile for Raspberry Pi 5 (Cortex-A76)
+#   make T=milkv                  - Cross-compile for Milk-V Pioneer (RISC-V 64)
 #   make help                     - Show all available targets
 #
 # ==============================================================================
@@ -23,10 +25,8 @@ CC = gcc
 # See docs/DEVELOPER.md for installation instructions
 ISABELLE ?= isabelle
 
-# Architecture / target selection — T= is a shorthand for ARCH= (board/SBC)
-ifdef T
-ARCH ?= $(T)
-endif
+# Board / architecture selection — T= picks a board (see mk/boards.mk)
+include mk/boards.mk
 ARCH ?= $(shell uname -m)
 ARCH := $(strip $(ARCH))
 TARGET := fastest
@@ -105,6 +105,33 @@ else
     ASM_SYNTAX =
 endif
 
+# Board-specific overrides — refine arch defaults for exact hardware
+# QEMU: use portable baseline, not -march=native
+ifeq ($(BOARD),qemu)
+    ARCH_FLAGS   := -march=x86-64 -mtune=generic
+    ARCH_DEFINES += -DBOARD_QEMU=1
+# PC: native tuning (already the amd64 default; explicit for clarity)
+else ifeq ($(BOARD),pc)
+    ARCH_FLAGS   := -march=native
+    ARCH_DEFINES += -DBOARD_PC=1
+# Raspberry Pi 4 — Cortex-A72, ARMv8-A
+else ifeq ($(BOARD),pi4)
+    ARCH_FLAGS   := -march=armv8-a+crc+simd -mtune=cortex-a72
+    ARCH_DEFINES += -DBOARD_RPI4=1 -DRASPBERRY_PI_BUILD=1
+# Raspberry Pi 5 — Cortex-A76, ARMv8.2-A
+else ifeq ($(BOARD),pi5)
+    ARCH_FLAGS   := -march=armv8.2-a+crypto+fp16+rcpc+dotprod -mtune=cortex-a76
+    ARCH_DEFINES += -DBOARD_RPI5=1 -DRASPBERRY_PI_BUILD=1
+# Milk-V Pioneer — SG2042 RISC-V 64
+else ifeq ($(BOARD),milkv)
+    ARCH_FLAGS   := -march=rv64gc -mabi=lp64d -mcmodel=medany
+    ARCH_DEFINES += -DBOARD_MILKV=1
+endif
+# Use board name in ARCH_NAME when T= is set
+ifdef BOARD_NAME
+    ARCH_NAME := $(BOARD_NAME)
+endif
+
 # Compiler flags
 # Feature switches (default: enabled)
 # Physics/SSM knob family: repointed through the Kconfig bridge (see
@@ -1251,18 +1278,19 @@ help:
 	@echo "════════════════════════════════════════════════════════════"
 	@echo ""
 	@echo "🛠️  Variables:"
-	@echo "  ARCH=<arch>      (default: $(ARCH); canonical directory: $(ARCH_DIR))"
-	@echo "  TARGET=<profile> (standard | fast | fastest | turbo)"
+	@echo "  T=<board>        Board target (preferred). Supported:"
+	@echo "                     qemu    — portable x86-64 for QEMU"
+	@echo "                     pc      — x86-64 PC (native tuning)"
+	@echo "                     pi4     — Raspberry Pi 4 (Cortex-A72)"
+	@echo "                     pi5     — Raspberry Pi 5 (Cortex-A76)"
+	@echo "                     milkv   — Milk-V Pioneer (SG2042 RISC-V 64)"
+	@echo "  ARCH=<arch>      Low-level arch override (amd64 | arm64 | riscv64)"
+	@echo "                   Active: $(ARCH) / board: $(or $(BOARD_NAME),$(ARCH_NAME))"
 	@echo ""
-	@echo "🏆 OPTIMIZATION BUILDS:"
-	@echo "  fastest         - Maximum performance (ASM + direct threading + LTO)"
-	@echo "  fast            - Fast without LTO (easier debugging)"
-	@echo "  turbo           - Assembly optimizations only"
+	@echo "🏆 BUILD PROFILE: fastest (always — ASM + direct threading + LTO)"
 	@echo ""
 	@echo "🥧 PLATFORM-SPECIFIC BUILDS:"
-	@echo "  rpi4            - Native build on Raspberry Pi 4"
-	@echo "  rpi4-cross      - Cross-compile from x86_64"
-	@echo "  rpi4-fastest    - Maximum optimization for RPi4"
+	@echo "  rpi4-cross      - Cross-compile for Raspberry Pi 4 (legacy; use T=pi4)"
 	@echo "  riscv64-clang   - Cross-compile for RISC-V 64 via clang (GCC fails here)"
 	@echo "  minimal         - Minimal/embedded build"
 	@echo ""