Scope prepare-gems dep to docs goals only

Per @chatgpt-codex-connector review on #4: making ext/configure-ext.mk
unconditionally depend on prepare-gems meant plain \`make\`/\`main\`
builds would chain into update-gems, which fetches bundled-gem
archives over the network. That breaks offline builds on a fresh
checkout and mutates the source tree in a way unrelated to the
requested target.

Move the dep to defs/gmake.mk under a MAKECMDGOALS filter for the
docs targets (rdoc, rdoc:%, html, html-server, rdoc-coverage), which
are the only ones whose recipes need bundled-gem extensions
activatable. Plain \`make\` no longer pulls the prepare-gems chain.

Verified:
- \`make -n html\` shows "Extracting bundle gem ..." lines
- \`make -n\` (plain) shows no extraction or download
- \`rm -rf .bundle && make html\` still completes (13895 items)

https://claude.ai/code/session_01YRoRyZPew2LtN6u6BX8bzx

Author: claude

common.mk

@@ -346,8 +346,7 @@ $(EXTS_MK): ext/configure-ext.mk $(srcdir)/template/exts.mk.tmpl \
 	    $(srcdir)/template/exts.mk.tmpl --gnumake=$(gnumake) --configure-exts=ext/configure-ext.mk
 
 ext/configure-ext.mk: $(PREP) all-incs $(MKFILES) $(RBCONFIG) $(LIBRUBY) \
-		$(srcdir)/template/configure-ext.mk.tmpl update-default-gemspecs \
-		$(HAVE_BASERUBY:yes=prepare-gems)
+		$(srcdir)/template/configure-ext.mk.tmpl update-default-gemspecs
 	$(ECHO) generating makefiles $@
 	$(Q)$(MAKEDIRS) $(@D)
 	$(Q)$(MINIRUBY) $(tooldir)/generic_erb.rb -o $@ -c \

defs/gmake.mk

@@ -358,6 +358,15 @@ $(srcdir)/gems/%.gem:
 extract-gems: | $(patsubst %,$(srcdir)/.bundle/gems/%,$(bundled-gems))
 extract-gems: | $(call foreach-bundled-gems-rev,bundled-gem-extracted)
 
+# When building docs, ensure bundled gems are extracted before
+# ext/configure-ext.mk is generated, since its template enumerates
+# bundled-gem extensions via Dir.glob(".bundle/gems/**/extconf.rb").
+# Scoped to docs goals so plain `make` doesn't reach for the network
+# via prepare-gems -> update-gems on a fresh checkout.
+ifneq ($(filter rdoc rdoc\:% html html-server rdoc-coverage,$(MAKECMDGOALS)),)
+ext/configure-ext.mk: $(HAVE_BASERUBY:yes=prepare-gems)
+endif
+
 $(srcdir)/.bundle/gems/%: $(srcdir)/gems/%.gem | .bundle/gems
 	$(ECHO) Extracting bundle gem $*...
 	$(Q) $(BASERUBY) -C "$(srcdir)" \