feat(stellar): preflight network checks before deploy

[gloriaelishahabu]

Summary

Adds a standalone preflight validation script for Soroban network deployments and
hooks it into deploy.sh as the first step, per #63.

Currently deploy.sh accepts testnet|futurenet|mainnet but never validates that
the network is actually usable before starting a deploy — a bad RPC URL, an
unreachable friendbot, or a missing identity only surfaces mid-deploy with a
confusing error. This adds a fast-failing check up front.

Closes #63

Changes

• stellar/scripts/check-network.sh — new standalone script, callable
  independently or as a preflight step. For the given network it verifies:

  • network passphrase is configured
  • RPC endpoint responds to a getHealth call
  • friendbot is reachable (testnet/futurenet only — skipped for mainnet)
  • the deploying identity exists locally
  • any network-specific required env vars are set (e.g. mainnet account)

  Prints a summary table of each check and exits non-zero if any check fails,
  with a clear error message per failure.

• stellar/deploy.sh — now calls check-network.sh "$NETWORK" as the very
  first step and aborts before doing any work if preflight fails.

• .github/workflows/check-network-test.yml — CI test that runs the script
  against an invalid network name (expects non-zero exit) and against testnet
  (expects it to run and print the summary table).

• stellar/README.md — documents the new script and usage.

How to test

./stellar/scripts/check-network.sh testnet
./stellar/scripts/check-network.sh mainnet
./stellar/scripts/check-network.sh badnet   # should fail with usage error
./stellar/deploy.sh testnet                 # preflight now runs first

Notes / follow-ups

• RPC URLs and passphrases are currently hardcoded in the script; happy to move
  these to a shared config file if the maintainers prefer a single source of
  truth with deploy.sh.
• Identity check uses stellar keys address <name>; let me know if the project
  has a different convention for resolving the deploying identity.

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