Add MORE_STORES.md, and close the builder image item

MORE_STORES.md asks which further host platforms could get a store like
the Batocera one, and starts from a measurement: the catalog ships html
for 11 of 15 titles and desktop binaries for 8, while the Batocera store
consumes only cartridge — 7. The gap is not the number of stores but
that the existing one leaves half the catalog untouched.

Also records that the ARM64 asset kind is the prerequisite for the
retro-handheld direction, and closes the internal-org item in
REPO_REFACT.md now that all eight builder images live under build/.
The Woodpecker note is corrected against what the CLI actually reports:
19 repos registered and active, not the 10 the pipelines table shows.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -80,6 +80,9 @@ any single repo:
   is for and what it contains.
 - **[ECOSYSTEM_PLAN.md](ECOSYSTEM_PLAN.md)** — what we have per org towards a
   complete game development lifecycle, and what is still missing.
+- **[MORE_STORES.md](MORE_STORES.md)** — which further host platforms could get
+  a store like the Batocera one, measured against what the catalog actually
+  ships.
 
 The `ttg-ops` plugin in `ttg-marketplace` reads these rather than carrying its
 own copy of the rules.