Path B pivot + Phase 0-3 closed with first baseline numbers

This is a from-scratch initial commit on a fresh .git. The original
scaffold commit (7510b56) and the earlier session's working-tree
docs were lost in a 2026-05-18 10:25 working-tree wipe; the corrupted
.git is preserved at .git-broken-2026-05-18/ (gitignored) for
forensic inspection.

Scope re-anchored from Path A (custom VPU firmware on VC7 scalar
cores; blocked by BCM2712 silicon-RoT mask-ROM signature check)
to Path B (QPU compute kernels via Mesa v3d / Vulkan compute or
direct DRM, on stock signed Pi 5 / CM5). See README.md and
docs/phase0.md for the substrate audit that closed Path A.

Phases closed:
  Phase 0 — substrate audit; Path A blocked, Path B open;
            codec-back-end-fits-QPU finding (docs/phase0.md)
  Phase 1 — first kernel locked (VP9 / AV1 8x8 inverse DCT) with
            publish-before-measure R = M2/M3 decision rules
            (docs/phase1.md)
  Phase 2 — reference impls mapped; FFmpeg n7.1.3 source vendored
            under external/ffmpeg-snapshot/ (PROVENANCE.md pins
            commit f46e514 + per-file SHA-256s) (docs/phase2.md)
  Phase 3 — real baseline measurements on hertz (docs/phase3.md):
              M1 bit-exact            100.0000 % (10000/10000)
              M3 NEON IDCT8 single    8.171 Mblock/s (122.4 ns/block)
              M5a empty Vulkan submit 22.66 us
              M5b 1-WG noop dispatch  55.60 us
              M5 delta                32.95 us/dispatch
            => per-dispatch overhead is ~455x per-NEON-block cost;
               Phase 4 must batch at frame level or close to it.

Build harness in place: CMakeLists.txt + tests/{bench_neon_idct.c,
vp9_idct8_ref.c, bench_vulkan_dispatch.c, shaders/noop.comp} +
external/ffmpeg-snapshot/config.h shim (7 defines + EXTERN_ASM).
Builds clean on Debian Trixie aarch64 with cmake 3.31, ninja 1.12,
libvulkan-dev 1.4.309, glslang-tools 15.1.0. Vendored FFmpeg .S
assembles via the config.h shim.

Next: Phase 4 (plan first QPU IDCT kernel under the M5 batching
constraint) -> Phase 5 second-model review -> Phase 6 implement.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -0,0 +1,103 @@
+# daedalus-fourier — Phase 3 baseline + (later) Phase 6 implementation.
+#
+# Builds:
+#   bench_neon_idct  — NEON throughput baseline (Phase 3 M3) +
+#                      bit-exact correctness gate (Phase 1 M1).
+#   bench_vulkan_dispatch — Vulkan compute dispatch-overhead baseline (M5).
+#
+# Linkage note: bench_neon_idct statically links the vendored
+# FFmpeg n7.1.3 NEON snapshot (LGPL-2.1+); see
+# external/ffmpeg-snapshot/PROVENANCE.md.
+
+cmake_minimum_required(VERSION 3.20)
+project(daedalus-fourier C ASM)
+
+set(CMAKE_C_STANDARD 11)
+set(CMAKE_C_STANDARD_REQUIRED ON)
+
+if (NOT CMAKE_BUILD_TYPE)
+    set(CMAKE_BUILD_TYPE Release)
+endif()
+
+if (NOT CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64")
+    message(FATAL_ERROR
+        "daedalus-fourier targets aarch64 (Pi 5 / BCM2712). "
+        "Cross-compile not yet wired.")
+endif()
+
+add_compile_options(-Wall -Wextra -Wno-unused-parameter)
+
+# ---- Vendored FFmpeg snapshot (LGPL-2.1+) -----------------------------------
+
+set(FFSNAP ${CMAKE_SOURCE_DIR}/external/ffmpeg-snapshot)
+
+# Assembly preamble (config.h shim + FFmpeg's asm helpers) used by the
+# vendored .S file. -I flags expose:
+#   - FFSNAP/                    so `#include "config.h"` finds our shim
+#   - FFSNAP/libavcodec/aarch64/ so `#include "neon.S"` finds the helper
+#   - FFSNAP/                    so `#include "libavutil/aarch64/asm.S"`
+#                                resolves against the vendored copy
+set(FFASM_FLAGS
+    -I${FFSNAP}
+    -I${FFSNAP}/libavcodec/aarch64
+    -I${FFSNAP}
+)
+
+set(FFASM_SOURCES
+    ${FFSNAP}/libavcodec/aarch64/vp9itxfm_neon.S
+)
+
+# Tell CMake/gas to preprocess .S sources.
+set_source_files_properties(${FFASM_SOURCES} PROPERTIES
+    COMPILE_OPTIONS "${FFASM_FLAGS}"
+    LANGUAGE ASM)
+
+# ---- NEON baseline microbench ----------------------------------------------
+
+add_executable(bench_neon_idct
+    tests/bench_neon_idct.c
+    tests/vp9_idct8_ref.c
+    ${FFASM_SOURCES}
+)
+target_compile_options(bench_neon_idct PRIVATE -O3 -march=armv8-a+simd)
+# bench_neon_idct doesn't need vulkan/drm — pure CPU baseline.
+
+# ---- Vulkan dispatch-overhead microbench (next chunk) ----------------------
+# Stub: written in a follow-up step. Toggle ON with -DDAEDALUS_BUILD_VULKAN=ON
+# once tests/bench_vulkan_dispatch.c exists.
+
+option(DAEDALUS_BUILD_VULKAN "Build Vulkan compute-dispatch microbench" ON)
+
+if (DAEDALUS_BUILD_VULKAN)
+    find_package(Vulkan REQUIRED)
+
+    # Compile GLSL compute shaders to SPIR-V via glslangValidator.
+    # The binary loads them at runtime from the build dir (cwd-relative).
+    find_program(GLSLANG_VALIDATOR
+        NAMES glslangValidator glslang
+        REQUIRED)
+
+    set(NOOP_SPV ${CMAKE_BINARY_DIR}/noop.spv)
+    add_custom_command(
+        OUTPUT ${NOOP_SPV}
+        COMMAND ${GLSLANG_VALIDATOR} -V -o ${NOOP_SPV}
+                ${CMAKE_SOURCE_DIR}/tests/shaders/noop.comp
+        DEPENDS ${CMAKE_SOURCE_DIR}/tests/shaders/noop.comp
+        COMMENT "glslang: noop.comp -> noop.spv"
+        VERBATIM
+    )
+    add_custom_target(daedalus_shaders ALL DEPENDS ${NOOP_SPV})
+
+    add_executable(bench_vulkan_dispatch tests/bench_vulkan_dispatch.c)
+    add_dependencies(bench_vulkan_dispatch daedalus_shaders)
+    target_link_libraries(bench_vulkan_dispatch PRIVATE Vulkan::Vulkan)
+    target_compile_options(bench_vulkan_dispatch PRIVATE -O2)
+endif()
+
+# ---- Summary ----------------------------------------------------------------
+
+message(STATUS "daedalus-fourier build configured for ${CMAKE_SYSTEM_PROCESSOR}")
+message(STATUS "  FFmpeg snapshot: ${FFSNAP}")
+message(STATUS "  Build type:      ${CMAKE_BUILD_TYPE}")
+message(STATUS "  Targets:         bench_neon_idct"
+               "$<$<BOOL:${DAEDALUS_BUILD_VULKAN}>:; bench_vulkan_dispatch>")