pkg: wire top-level CMake, clang-format/tidy, editorconfig

CMakeLists.txt: every lane's add_subdirectory() is now guarded by EXISTS
on that lane's CMakeLists.txt, so main configures no matter which lanes
have merged and a lane lights up its targets on merge with no edit here.
Dependencies are found at top level (gated on the consuming lane) so a
missing -dev package fails fast with a clear name. Warnings via
add_compile_options (survives the presets' CMAKE_CXX_FLAGS override);
VELOX_WERROR escape hatch. Verified end to end: `cmake --preset dev`
against a merged lane/core builds libveloxcore + tests, `ctest --preset
dev` green.

.clang-format: Google base, 4-space indent, 100 cols, right-aligned
pointers. .clang-tidy: small high-signal set (bugprone/performance/
concurrency/portability + selected modernize/readability). .editorconfig
mirrors both. .gitignore: build-*/ , CMakeUserPresets.json, profiling
and editor droppings.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01HPPSGhiArbvQgwC2DNiURS

.editorconfig

@@ -0,0 +1,39 @@
+# Velox editor defaults. Owned by lane PKG/QA. Mirrors .clang-format for C++ and the
+# prevailing conventions for everything else.
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+indent_style = space
+
+[*.{cpp,cc,hpp,h}]
+indent_size = 4
+max_line_length = 100
+
+[*.{ts,tsx,js,jsx,json,mjs}]
+indent_size = 2
+
+[*.{py}]
+indent_size = 4
+max_line_length = 100
+
+[*.{cmake,txt}]
+indent_size = 4
+
+[CMakeLists.txt]
+indent_size = 4
+
+[*.{yml,yaml}]
+indent_size = 2
+
+[*.sh]
+indent_size = 4
+
+[*.md]
+trim_trailing_whitespace = false
+
+[Makefile]
+indent_style = tab