fix(ecs): thread index declarations into the computed-factory db type

[krisnye]

Summary

db.indexes was empty inside computed factories — computeds couldn't use any declared index.

Database is 9 type params:

Database<C, R, A, F, S, AF, SV, CV, IX extends IndexDeclarations<C>>
//        1  2  3  4  5  6   7   8   9

The actions/systems db correctly threads slot 9 (IX). But the computed-factory db is FullDBForPlugin<…>, which forwarded only slots 1–7 — so CV (8) and IX (9) fell to their defaults, leaving IX = {} and db.indexes empty.

Fix

Forward IX into slot 9 of FullDBForPlugin, supplying a placeholder for slot 8:

type FullDBForPlugin<…, SVF, IX = {}> =
  Database<…,
    FromServiceFactories<RemoveIndex<SVF> & XP['services']>,
    unknown,             // 8: CV — Database's own default for this slot, so a
                         //    concrete computed-values type never constrains
                         //    computed-factory inference / breaks contravariance
    IX & XP['indexes']>; // 9: IX — the missing thread

At the call sites XP is AmbientPlugin<XP, IP>, so XP['indexes'] resolves to extends-base and imports-dep indexes; the IX param adds the plugin's own locally-declared indexes. This matches the actions/systems db exactly.

Verification

• tsc -b green; 210 index/plugin tests pass; lint clean.
• Added regression guards in database.index.type-test.ts proving a computed factory sees its own, its extends-base, and its imports-dep indexes via db.indexes. Confirmed they fail to compile without the fix (Property '<name>' does not exist on type '{}').
• Pure type-threading change; no runtime change.

Bump to 0.9.66.

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