feat: Use arm64 pic binaries

[adamspofford-dfinity]

Since arm64 and x64 seem to have incompatible state archives, replaces the preinitialized state in tests with PocketIC's native feature for setting up the NNS.

[Copilot]

Pull request overview

This PR updates the PocketIC binary download logic to support arm64, and refactors the nns_proxy example tests to stop relying on a pre-initialized NNS state archive by using PocketIC’s native NNS setup + ICP feature configuration.

Changes:

• Add arch-aware platform selection for downloading the correct PocketIC binary (arm64 vs x86_64).
• Remove repo-level NNS state archive extraction and switch nns_proxy tests to SubnetStateType.New + icpFeatures-based NNS/ledger setup.
• Remove the hard-coded “minter” test identity and mint test funds from the anonymously-seeded ICP ledger instead.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
packages/pic/postinstall.mjs	Selects PocketIC download artifact based on CPU architecture.
package.json	Removes state-archive extraction from scripts; simplifies setup.
examples/nns_proxy/tests/src/support/ledger.ts	Mints funds by transferring from the anonymously seeded ICP balance.
examples/nns_proxy/tests/src/support/identity.ts	Deletes hard-coded minter key material used for minting in tests.
examples/nns_proxy/tests/src/support/governance.ts	Parameterizes neuron stake and adjusts mint/stake amounts for neuron creation.
examples/nns_proxy/tests/src/nns-proxy.spec.ts	Uses fresh NNS state + enables required ICP features; adds “ballast” neuron approach.

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