Merge branch 'main' into golangci-v2.12.2

Absorbs the cache size management and LRU eviction work (#55). All four
textual conflicts resolved in favor of main's implementation, with this
branch's mechanical lint conformance re-applied on top:

- internal/config/config.go: took main's cache_max_bytes wiring
  (CacheMaxBytes, cacheMaxBytesExplicit) verbatim and expressed the key
  through this branch's constant convention as keyCacheMaxBytes.
- internal/imgcache/cache.go: took main's disabled-cache guards, LRU
  touch on lookup, and variant_content size accounting verbatim;
  re-applied this branch's signature wrapping for lll.
- internal/imgcache/storage.go: took main's new writeIfAbsent helper and
  its temp-file cleanup defer verbatim. The auto-merge had silently
  dropped that defer in favor of this branch's inline cleanup form;
  restored so the cleanup semantics that arrived from main are intact.
- TODO.md: kept both sides' Completed Steps entries.

.golangci.yml resolves to this branch's canonical version
(sha256 021cc83f4e6fc7c31b95b34b846723dfcf20b66b7baeea1dc40406e643346bcb).

make build and make test are green; #55's code is not yet conformant
with the canonical lint config, which the following commits address.

script/test

@@ -17,7 +17,7 @@ run_with_cgo_deps() {
 main() {
     cd "$ROOT"
     echo "Running tests..."
-    run_with_cgo_deps "CGO_ENABLED=1 go test -timeout 30s -v ./..."
+    run_with_cgo_deps "CGO_ENABLED=1 go test -timeout 30s -race -v ./..."
 }
 
 main "$@"