Update stale design-tradeoff: cache shipped, not TODOHEADmaster

The shared-deps tradeoff bullet still described the package cache as a
future TODO not yet implemented. The cache has shipped; point the bullet
at it instead.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

1 files changed, 6 insertions, 6 deletions

diff --git a/README.md b/README.md
index 39ba5cb..d5f70b5 100644
--- a/README.md
+++ b/README.md
@@ -168,12 +168,12 @@ overlay is always disposed and there is no keep-overlay option.
 
 ## Design tradeoffs (on the record)
 
-- **Shared deps are rebuilt per target.** Each category target starts from a
-  clean base, so a dep shared by N targets is rebuilt N times. Correct for
-  isolation, slow for heavy shared deps (qt5, boost). A future optional
-  package-output cache (keep built `.tgz` outside the overlay, `installpkg` a
-  cached dep when its SlackBuild + version are unchanged) is marked TODO in the
-  source, not yet implemented.
+- **Shared deps are rebuilt per target unless cached.** Each category target
+  starts from a clean base, so a dep shared by N targets would be rebuilt N
+  times. Correct for isolation, slow for heavy shared deps (qt5, boost). The
+  optional package cache (see above) mitigates this: a dep is installed from
+  cache when its version is unchanged instead of rebuilt. The named target
+  always builds fresh regardless.
 - **overlayfs sharp edges.** A build that fails ONLY in the overlay but works on
   bare 15.0 is more likely an overlayfs quirk (rename/whiteout) than a real
   build bug. Noted near the build step in the source.