Build bundled-gem extensions for docs targets

After b5e6e0a (Use rbs-integrated RDoc), tool/rdoc-srcdir does
require 'rdoc/rdoc' which activates rdoc-7.2.0 and its new
add_dependency 'rbs', '>= 4.0.0'. RubyGems then needs rbs's gemspec
in .bundle/specifications/ and its C extension built. ext/extmk.rb
already handles both via each per-gem Makefile (all -> install ->
gemspec copies .bundled.rbs-*.gemspec into specifications/, and the
TARGET_SO recipe writes gem.build_complete) -- but only if
ext/configure-ext.mk has seen .bundle/gems/ populated when its
template's Dir.glob(".bundle/gems/**/extconf.rb") runs.

On a clean tree, ext/configure-ext.mk regenerates before
prepare-gems extracts anything, so its template glob finds nothing,
exts.mk lacks rules for bundled-gem extensions, the rbs C extension
never compiles, and RubyGems aborts with "Ignoring rbs-4.0.2 because
its extensions are not built."

Add prepare-gems as a prereq of ext/configure-ext.mk, scoped via
MAKECMDGOALS to docs goals only (rdoc, rdoc:%, html, html-server,
rdoc-coverage, undocumented). Plain `make` is untouched, so an
offline-fresh-checkout build doesn't reach prepare-gems -> update-gems.

Also fix tool/update-deps: with the new dep, `make -p all` now
includes rules from inside .bundle/gems/<gem>/ext/<ext>/, where make
prints relative targets like `bigdecimal.o:` whose curdir is under
.bundle/. The existing filter checked the raw target string, missing
these; move the check onto `dir + target` so the full path is what's
inspected. Bundled gems own their depend files, so skipping them here
matches the intent of the original filter.

https://claude.ai/code/session_01YRoRyZPew2LtN6u6BX8bzx

Author: claude

defs/gmake.mk

@@ -358,6 +358,21 @@ $(srcdir)/gems/%.gem:
 extract-gems: | $(patsubst %,$(srcdir)/.bundle/gems/%,$(bundled-gems))
 extract-gems: | $(call foreach-bundled-gems-rev,bundled-gem-extracted)
 
+# Docs targets call $(RDOC) -> tool/rdoc-srcdir, which loads rdoc/rdoc and
+# activates its `add_dependency 'rbs', '>= 4.0.0'`. Activation requires
+# the bundled rbs gem to be discoverable and its C extension built. The
+# existing per-bundled-gem Makefile generated by ext/extmk.rb does both
+# (extension build + copy of .bundled.rbs-*.gemspec into
+# .bundle/specifications/), but only if ext/configure-ext.mk has seen
+# `.bundle/gems/` populated when its template's
+# `Dir.glob(".bundle/gems/**/extconf.rb")` runs. On a clean tree
+# prepare-gems may not have run yet, so make the dep explicit for the
+# docs goals. Other targets are left alone so a plain `make` doesn't
+# reach for prepare-gems -> update-gems (network).
+ifneq ($(filter rdoc rdoc:% html html-server rdoc-coverage undocumented,$(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)" \

tool/update-deps

@@ -328,12 +328,13 @@ def read_make_deps(cwd)
       next if /libyjit.o\z/ =~ target.to_s # skip YJIT Rust object (no corresponding C source)
       next if /libzjit.o\z/ =~ target.to_s # skip ZJIT Rust object (no corresponding C source)
       next if /target\/release\/libruby.o\z/ =~ target.to_s # skip YJIT+ZJIT Rust object (no corresponding C source)
-      next if /\.bundle\// =~ target.to_s
       next if /\A\./ =~ target.to_s # skip rules such as ".c.o"
       #p [curdir, target, deps]
       dir = curdir || dirstack.last
-      dependencies[dir + target] ||= []
-      dependencies[dir + target] |= deps.map {|dep| dir + dep }
+      key = dir + target
+      next if /\.bundle\// =~ key.to_s # bundled gem extensions manage their own depend files
+      dependencies[key] ||= []
+      dependencies[key] |= deps.map {|dep| dir + dep }
     elsif data_base_end
       curdir = nil
     elsif directory_leave