fix: Cross-Platform build fails — missing provable-contracts sibling checkout

[noahgift]

Summary

Cross-Platform Build workflow fails on all platforms:

```
error: failed to load manifest for workspace member `/home/runner/work/presentar/presentar/crates/presentar-core`
failed to load manifest for dependency `provable-contracts-macros`
failed to read `/home/runner/work/presentar/provable-contracts/crates/provable-contracts-macros/Cargo.toml`
No such file or directory (os error 2)
```

Five-Whys

1. Why does Cross-Platform fail? — `provable-contracts-macros` sibling path dep can't be read
2. Why? — Cross-Platform workflow doesn't check out the provable-contracts sibling repo
3. Why? — Workflow was written before provable-contracts was added as a path dep
4. Why wasn't workflow updated? — No automation cross-references Cargo.toml path deps with workflow checkout steps
5. Why no template? — sovereign-ci shared workflow doesn't yet expose a composite sibling-checkout step

Fix (proven pattern from aprender/forjar/copia)

```yaml

• name: Checkout provable-contracts (path dep)
  uses: actions/checkout@11bd719
  with:
  repository: paiml/provable-contracts
  path: provable-contracts

• name: Symlink provable-contracts for Cargo path deps
  if: runner.os != 'Windows'
  run: ln -sf "$GITHUB_WORKSPACE/provable-contracts" "$GITHUB_WORKSPACE/../provable-contracts"

• name: Symlink provable-contracts (Windows)
  if: runner.os == 'Windows'
  shell: pwsh
  run: New-Item -ItemType Junction -Path "$env:GITHUB_WORKSPACE\..\provable-contracts" -Target "$env:GITHUB_WORKSPACE\provable-contracts" -Force
  ```

Refs paiml/infra#15