feat(flash): pm3flash --build PLATFORM (build from the submodule, then flash)

build_firmware()/find_firmware_src() cross-compile fullimage.elf via
'make -C firmware PLATFORM=<explicit> armsrc/all' and pm3flash --build flashes it.
PLATFORM is explicit (never inferred from is_rdv4 — that's the running firmware's
flag, not the board).

Hardened per an adversarial multi-agent review of the change:
- --build now implies --force: it must flash the image it just built. A dirty
  rebuild keeps the same base SHA, so matches_pin can't distinguish it from the
  old firmware and would otherwise silently skip the flash (exit 0).
- make missing/not-executable -> FlashError (was an uncaught OSError traceback).
- find_firmware_src no longer falls back to a CWD-relative ./firmware — running
  make on a stray Makefile is arbitrary code execution; only explicit arg /
  $PM3PY_FIRMWARE_SRC / the package repo root. Suite 1045 pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_flash.py

@@ -12,10 +12,23 @@ from pm3py.core.protocol import (
     DEVICE_INFO_FLAG_CURRENT_MODE_BOOTROM, DEVICE_INFO_FLAG_CURRENT_MODE_OS,
 )
 from pm3py.core.transport import PM3Response, encode_old_frame, decode_old_frame
+from pm3py.core import flash as flashmod
 from pm3py.core.flash import (
     Flasher, FlashError, parse_elf_segments, build_blocks, resolve_image,
+    find_firmware_src, build_firmware,
 )
 
+
+def _proc(returncode):
+    return type("P", (), {"returncode": returncode})()
+
+
+def _fake_src(tmp_path):
+    (tmp_path / "Makefile").write_text("x")
+    (tmp_path / "armsrc").mkdir()
+    (tmp_path / "armsrc" / "Makefile").write_text("x")
+    return tmp_path
+
 OS_FLAGS = (DEVICE_INFO_FLAG_BOOTROM_PRESENT | DEVICE_INFO_FLAG_OSIMAGE_PRESENT
             | DEVICE_INFO_FLAG_CURRENT_MODE_OS)
 BOOTROM_FLAGS = (DEVICE_INFO_FLAG_BOOTROM_PRESENT | DEVICE_INFO_FLAG_OSIMAGE_PRESENT
@@ -160,6 +173,74 @@ def test_resolve_image_none_no_build(tmp_path, monkeypatch):
         resolve_image(None)
 
 
+# --- build_firmware / find_firmware_src ---
+
+def test_find_firmware_src_env(tmp_path, monkeypatch):
+    src = _fake_src(tmp_path)
+    monkeypatch.setenv("PM3PY_FIRMWARE_SRC", str(src))
+    assert find_firmware_src() == src
+
+
+def test_build_firmware_invalid_platform():
+    with pytest.raises(FlashError, match="PLATFORM"):
+        build_firmware("PM3; rm -rf /")
+
+
+def test_build_firmware_runs_make(tmp_path, monkeypatch):
+    src = _fake_src(tmp_path)
+    obj = src / "armsrc" / "obj"
+    obj.mkdir()
+    (obj / "fullimage.elf").write_bytes(b"\x7fELF")
+    seen = {}
+
+    def fake_run(cmd, *a, **k):
+        seen["cmd"] = cmd
+        return _proc(0)
+
+    monkeypatch.setattr(flashmod.subprocess, "run", fake_run)
+    elf = build_firmware("PM3GENERIC", firmware_dir=str(src))
+    assert elf == obj / "fullimage.elf"
+    # explicit platform, non-shell argv, armsrc/all target
+    assert seen["cmd"] == ["make", "-C", str(src), "PLATFORM=PM3GENERIC", "armsrc/all"]
+
+
+def test_build_firmware_make_fails(tmp_path, monkeypatch):
+    src = _fake_src(tmp_path)
+    monkeypatch.setattr(flashmod.subprocess, "run", lambda *a, **k: _proc(2))
+    with pytest.raises(FlashError, match="build failed"):
+        build_firmware("PM3RDV4", firmware_dir=str(src))
+
+
+def test_build_firmware_missing_source(monkeypatch):
+    monkeypatch.setattr(flashmod, "find_firmware_src", lambda fd=None: None)
+    with pytest.raises(FlashError, match="source not found"):
+        build_firmware("PM3GENERIC")
+
+
+def test_build_firmware_make_not_on_path(tmp_path, monkeypatch):
+    # make missing/not-executable -> FlashError, not a raw OSError traceback
+    src = _fake_src(tmp_path)
+
+    def boom(*a, **k):
+        raise FileNotFoundError(2, "No such file or directory", "make")
+
+    monkeypatch.setattr(flashmod.subprocess, "run", boom)
+    with pytest.raises(FlashError, match="could not run make"):
+        build_firmware("PM3GENERIC", firmware_dir=str(src))
+
+
+def test_find_firmware_src_ignores_cwd(tmp_path, monkeypatch):
+    # A firmware/ in the CWD must NOT be picked up — build_firmware runs `make` on
+    # the result, so a stray ./firmware would execute an untrusted Makefile.
+    cwd_fw = tmp_path / "firmware"
+    cwd_fw.mkdir()
+    _fake_src(cwd_fw)
+    monkeypatch.chdir(tmp_path)
+    monkeypatch.delenv("PM3PY_FIRMWARE_SRC", raising=False)
+    monkeypatch.setattr(flashmod, "_PACKAGE_ROOT", tmp_path / "nonexistent")
+    assert find_firmware_src() is None
+
+
 # --- Flasher: probing ---
 
 def test_device_info_parses_flags():