X_SIDE_EFFECTS_MISSING
Means
a package declares no sideEffects at all
Typical cause
every module is retained in every bundle that imports one binding — measured 2026-08-21 with buildIslands: an island importing @ultimat3/time weighed 22,214 B, and 5,948 B once @ultimat3/core declared the field. A ratchet, because 24 of 30 packages were silent the day the rule landed; the list may shrink and may never grow
Fix
bun run side-effects --explain --json, copy that package's array into its package.json, then bun run scripts/side-effects.ts --unpin <packages/name>