fix(memfs): Fix enumeration and RemoveAll for prefix-named siblings

[mojatter]

Summary

In memfs, a directory's entries could vanish from enumeration, and RemoveAll could delete unrelated directories, when a sibling shares the target's name as a string prefix (e.g. dir0-tmp alongside dir0). All three bugs stem from confusing a string prefix with a path-segment prefix in memfs/store.go. Open/Stat/ReadFile of an exact path were unaffected — only enumeration and bulk removal.

• prefixKeys (backs ReadDir, and therefore fs.WalkDir): broke the scan at the first key not under dir/. A sibling like dir0-tmp sorts between dir0 and dir0/... ('-' 0x2d < '/' 0x2f), so the real children were never reached and ReadDir returned zero entries. Siblings are now skipped (continue); the scan stops only when the shared string prefix ends.
• prefixGlobKeys (backs Glob): same shape, same fix.
• removeAll (backs RemoveAll): deleted every key with the string prefix, so RemoveAll("dir0") also removed dir0-tmp, dir0_bak, etc. It now deletes only the directory itself and its true children (key == prefix || HasPrefix(key, prefix+"/")).

Surfaced via a downstream backend layered on wfs that uses fs.WalkDir/ReadDir: sibling index directories made a base directory invisible to enumeration under mem://, while osfs and s3 behaved correctly (they walk real directories / list a flat keyspace without the contiguity shortcut).

Test plan

• go test -race ./...
• staticcheck ./memfs/
• gosec ./memfs/
• New table-driven tests reproduce each bug (dash sibling fails before the fix; underscore / no-sibling controls pass)