feat(ecs): surface base-plugin computeds inside derived computed factories

[krisnye]

Description

FullDBForPlugin hardcoded the computed-factory db's computed slot (CV) to unknown, which erased the extends / imports base's already-resolved computeds — not just the current plugin's in-progress ones. As a result, a derived plugin's computed factory could not compose on a base plugin's computed.

This PR sets that slot to FromComputedFactories<XP['computed']>. XP is AmbientPlugin<XP, IP> at the call sites, so it surfaces the base + imports computeds (all fully constructed — no circularity). The current plugin's own CVF is never a parameter to the type, so in-progress siblings stay hidden — the same rule actions/systems already follow.

const base = createPlugin({
  resources: { n: { default: 10 } },
  computed: { doubled: (db) => Observe.withMap(db.observe.resources.n, (v) => v * 2) },
});

const derived = createPlugin({
  extends: base,
  computed: {
    // now fully typed — previously db.computed was `unknown`
    quadrupled: (db) => Observe.withMap(db.computed.doubled, (v) => v * 2),
  },
});

Also

• Added type-tests for both directions: base composition works; an in-progress sibling stays hidden.
• Dropped Database.Plugin.ToComputedDb (introduced earlier in this branch). It was a computed: unknown mirror of the old factory context; now that factories see the base's resolved computeds it no longer mirrors anything, and its residual "strip computed" use is a near-duplicate of ToDatabase. The rare call site can use Omit<ToDatabase<P>, 'computed'> inline.
• Version bump to 0.9.68.

Verification

• tsc -b clean (incl. @ts-expect-error assertions in the new tests)
• pnpm lint clean
• Full @adobe/data suite: 2584 passed