Cache scoped executor / catalog reads with short TTL

[amondnet]

Sub-task of #3 (Fix C).

createExecutor is rebuilt per request (packages/core/api/src/server/scoped-executor.ts:253; a TTL-cache TODO is noted in the code).

Tasks:

• Cache scoped executor / catalog + policy reads per (org, subject) with a few-second TTL.
• Ensure tenant isolation in the cache key.
• Largely amortizes fan-out + repeated navigation (partly subsumed by Fix B for first paint).

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