CI: find clang-c/Index.h and libclang where Ubuntu and Homebrew keep them; the language server on Windows paths

CI has failed since the embed merge (2026-10-25):
- Linux: bootstrap/build.sh asks llvm-config for the folder with clang-c/Index.h
- macOS: FINCH_LIBCLANG's -L folders also go to the compiler's own link "clang"
- Windows: the language server turns file:///C:/x into C:/x and back (diagnostics went to /C:/x);
  the -g tests compare without \r; shared.sh (dlopen, -rdynamic) is not run there

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016GzNP6r8dg6WDxUVZy8wjL

tests/lsp_test.py

@@ -84,7 +84,8 @@ init = c.request("initialize", {"processId": None, "rootUri": None, "capabilitie
 check("initialize", init["capabilities"]["hoverProvider"] is True)
 c.notify("initialized", {})
 c.notify("textDocument/didOpen", {"textDocument": {"uri": uri, "languageId": "finch", "version": 1, "text": src}})
-check("no errors in a good file", c.diagnostics(uri) == [])
+first = c.diagnostics(uri)
+check("no errors in a good file", first == [], first)
 
 h = c.request("textDocument/hover", {"textDocument": td, "position": pos(12, "total")})
 check("hover: variable type", h and "int total" in h["contents"]["value"], h)