feat: export computeTargetBalances and prod-network for yds consumption

[samsiegart]

fixes https://linear.app/agoric/issue/AGO-573/make-computeweightedtargets-and-prod-networkts-quickly-consumable-from

Summary

Implements AGO-573 by making the YMax target-balance logic and production network spec consumable outside this repo.

• Extracts computeWeightedTargets from services/ymax-planner into @agoric/portfolio-api as computeTargetBalances
• Moves prod-network.ts / network-spec.ts into @agoric/portfolio-api/src/network
• Keeps packages/portfolio-contract/tools/network/prod-network.ts as a compatibility re-export
• Adds packages/portfolio-contract/tools/target-balances.ts as a compatibility re-export
• Updates internal consumers to import from @agoric/portfolio-api
• Adds README notes for how YDS should consume the API next

Reviewer Notes

Most of the large diff is moved code, not new behavior:

• prod-network.ts is copied from portfolio-contract/tools/network into portfolio-api/src/network
• network-spec.ts is copied with import/type-source cleanup
• computeTargetBalances is the former computeWeightedTargets, adapted from a planner-private helper into an exported API

The old behavior coverage remains in services/ymax-planner/test/deposit-tools.test.ts