build: pick first matching ports/<name>/ per gem

When `conf.ports` was given multiple names, every matching ports/
directory for each gem was compiled -- fine for single-name builds
and for gems that have at most one matching port, but a footgun
once HAL-using gems start providing multiple alternative ports
(e.g. mruby-task with both posix and an alternative runloop).

Iterate the chain and break on the first match.  Existing builds
that pass at most one name per gem keep the same behavior;
`conf.ports :rp2040, :posix` now reads as "use rp2040 if a gem has
that port, otherwise posix" rather than "compile both".

ref #6825

Co-authored-by: Claude <noreply@anthropic.com>

lib/mruby/build.rb

@@ -185,10 +185,11 @@ module MRuby
     end
 
     # Set target port names for this build.
-    # Gems with matching ports/<name>/ directories will compile
-    # those platform-specific sources automatically.
+    # Each gem compiles the first matching ports/<name>/ directory;
+    # later names in the list act as fallbacks for gems that don't
+    # ship a port for the earlier names.
     #   conf.ports :esp32
-    #   conf.ports :rp2040, :posix
+    #   conf.ports :rp2040, :posix    # use rp2040 if available, else posix
     def ports(*names)
       @port_names = names.map { |n| n.to_s }
     end

lib/mruby/gem.rb

@@ -60,10 +60,16 @@ module MRuby
         @rbfiles = Dir.glob("#{@dir}/mrblib/**/*.rb").sort
         @objs = srcs_to_objs("src")
 
-        # Add platform-specific sources from ports/<name>/ directories
+        # Add platform-specific sources from the first matching
+        # ports/<name>/ directory.  effective_ports is a fallback
+        # chain: later names act as defaults for gems that don't ship
+        # a port for the earlier names.
         build.effective_ports.each do |port|
           port_dir = "#{@dir}/ports/#{port}"
-          @objs += srcs_to_objs("ports/#{port}") if File.directory?(port_dir)
+          if File.directory?(port_dir)
+            @objs += srcs_to_objs("ports/#{port}")
+            break
+          end
         end
 
         @test_preload = nil # 'test/assert.rb'