Add docs-tests harness coverage for Network Accounts snippets

## Context

The new [Network Accounts](docs/builder/smart-contracts/accounts/network-accounts.md) page documents deploying a network account under v0.15 (`AuthNetworkAccount` + `NetworkAccountNoteAllowlist`, replacing the removed `AccountStorageMode::Network`).

Its Rust snippets are grounded in — and lifted near-verbatim from — miden-client's shipped integration test `bin/integration-tests/src/tests/network_transaction.rs` (v0.15.2) and miden-standards v0.15.3, so they compile upstream. They are **not yet** covered by the local `docs-tests` validation harness.

## Task

Add a `crates/network-account-demo` to the docs-tests harness that lifts the page's snippets verbatim (wrapped in exported fns) and goes green under `cargo build --keep-going`, so future v0.16+ API drift on:

- `AuthNetworkAccount::with_allowed_notes` / `with_allowed_tx_scripts`
- `AccountBuilder` + `AuthNetworkAccount` + `BasicWallet` build path
- client `add_account` / `submit_new_transaction` deploy flow
- `P2idNote::script_root()` / `MintNote::script_root()` / `NoteScriptRoot`

is caught before it reaches published docs.

## Out of scope

- The page content itself (already authored/reviewed).
- The tutorials-repo divergence (network-transactions tutorial still references `AccountStorageMode::Network`) — tracked separately.

Examine the existing docs-tests harness structure and the network account page's Rust snippets. Create a new crate under `crates/network-account-demo` that exports functions wrapping the snippets. Ensure it compiles with `cargo build --keep-going`. Reference the integration test `bin/integration-tests/src/tests/network_transaction.rs` and miden-standards for correct API usage. Verify the build passes to confirm coverage.

索引モデルが issue の本文から書いたものです。