feat: add API surface tracking with api-extractor for v1.0.0

[code3ks]

• Mark internal implementation helpers with @internal TSDoc tag
  • Stellar: scalar utilities, cache, event filters, batch builder
  • EVM/Solana/CKB: builders, deployments, announcements
• Add Stellar multisig exports (buildMultisigStealthWithdraw, etc.)
• Install @microsoft/api-extractor with configs for all 6 entry points
• Generate API report snapshots in etc/*.api.md
• Add pnpm api:check script to validate API surface
• Update CI to fail on undocumented API changes
• Update companion packages peerDependencies to >=1.0.0 <2

Closes #124

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[truthixify]

The api:check failure in test (22) is a first-run setup thing on your side. api-extractor is diffing the current public API against a committed baseline at etc/*.api.md, but the baselines don't exist yet because this PR is what's introducing them.

To fix:

1. Run pnpm build locally. api-extractor's local mode auto-generates the initial baseline files (etc/sdk.api.md, etc/sdk-evm.api.md, etc/sdk-stellar.api.md, etc/sdk-solana.api.md, etc/sdk-ckb.api.md, etc/sdk-vault.api.md).
2. git add etc/ and commit them.
3. Force-push.

After that CI will pass because the baselines exist and match the code. Future PRs that change the public API will need to update these files in the same commit.

[code3ks]

CI passed succesfully now,you can review and merge. Thank you

[truthixify]

Cascade conflict from sdk#149 landing right before this. Rebase against develop and force-push, should slot in cleanly next.

[truthixify]

In. API-extractor plus baseline files lands. Future public-surface changes will get flagged automatically now, nice work.