Merge main into the MLT branch

main moved each tool's option table to file scope so that usage.cpp can
generate the usage message from it (#409), which conflicted with the
tables the MLT options had been added to. The three MLT options move to
the file-scope tables, under the same headings they were listed with
before: "Setting or disabling tile size limits" in tippecanoe and
tile-join, and "Output tile" in tippecanoe-overzoom.

Also combine the new radix-sort-test with the MLT tests in the test
target, add usage.o to the tools that gained MLT objects, keep both the
MLT=0 and the new docs CI jobs, and regenerate the man page for the
README changes, which the docs job now checks.

mvt.hpp

@@ -97,12 +97,21 @@ struct mvt_value {
 		long long sint_value;
 		bool bool_value;
 		int null_value;
+		// Initializing string_value initializes the union's full width, which
+		// the static_assert below checks. Setting only a narrower member (a
+		// double, say) would leave the remaining bytes indeterminate, and the
+		// implicit copy constructor copies the union as a whole, so those
+		// bytes get read even when they aren't the member in use.
 		struct {
 			size_t off;
 			size_t len;
-		} string_value;
+		} string_value = {0, 0};
 	} numeric_value;
 
+	static_assert(sizeof(numeric_value) == sizeof(numeric_value.string_value),
+		      "string_value must span the whole union, since its default member "
+		      "initializer is what initializes the union");
+
 	std::string get_string_value() const {
 		if (type == mvt_string) {
 			return std::string(*s, numeric_value.string_value.off, numeric_value.string_value.len);