Check install completeness before spawning the linker-script gcc

A half-extracted framework cache now names the missing path instead of
surfacing a gcc failure; pure reorder, the checks depend on nothing
the generation step produces.

esphome/build_gen/arduino8266.py

@@ -932,23 +932,16 @@ def write_project(paths: InstalledPaths, ccache: str | None) -> bool:
         raise EsphomeError(f"Invalid flash mode {flash_mode!r}")
     flash_ld_name = _flash_ld_name(board)
 
-    generate_ld_scripts(paths, config, flash_ld_name)
-
     sdk = framework / "tools" / "sdk"
     core_dir = framework / "cores" / "esp8266"
     variant_dir = framework / "variants" / board_build["variant"]
     src_dir = CORE.relative_src_path()
 
-    libraries = resolve_libraries(
-        framework,
-        pio_platform="espressif8266",
-        board_mcu="esp8266",
-        cache_key="arduino8266",
-    )
-
     if not src_dir.is_dir():
         # Generated project state, not install state: clean-all would not help
         raise EsphomeError(f"Generated source directory {src_dir} is missing")
+    # Completeness checks run before generate_ld_scripts spawns gcc so a
+    # half-extracted install names the missing path, not a gcc error.
     # A missing install directory would otherwise surface as a wall of
     # include errors; failing here names the path instead.
     include_dirs = [
@@ -974,6 +967,15 @@ def write_project(paths: InstalledPaths, ccache: str | None) -> bool:
             raise EsphomeError(
                 f"{_INCOMPLETE_INSTALL}: missing {required_file}; {_CLEAN_HINT}"
             )
+
+    generate_ld_scripts(paths, config, flash_ld_name)
+
+    libraries = resolve_libraries(
+        framework,
+        pio_platform="espressif8266",
+        board_mcu="esp8266",
+        cache_key="arduino8266",
+    )
     for lib in libraries:
         include_dirs += lib.include_dirs
 

tests/unit_tests/build_gen/test_arduino8266.py

@@ -1709,5 +1709,7 @@ def test_write_project_rejects_spaced_ldscript_override(tmp_path: Path) -> None:
     CORE.platformio_options = {"board_build.ldscript": "my script.ld"}
     paths = _make_framework(tmp_path)
     _set_flags()
+    # The completeness checks run first; give them a src dir to pass
+    CORE.relative_src_path().mkdir(parents=True, exist_ok=True)
     with pytest.raises(EsphomeError, match="Invalid flash linker script name"):
         arduino8266.write_project(paths, None)