fix: repair broken PoC links in writeups

[arunimshukla]

Summary

• point private bond and shielded pool links at the pocs repository's actual master branch
• fix the stale cross-chain spec path from pocs/approach-private-trade-settlement/... to pocs/private-trade-settlement/...
• keep the change limited to the four affected writeups

Verification

• confirmed ethsystems/pocs has master as its default branch and no main branch
• confirmed every replacement target resolves on the current pocs/master tree
• confirmed the branch is based on the current ethsystems/web@main commit
• final diff: 4 files, 1 commit, URL-only changes

[rymnc]

hi @arunimshukla , thank you for your PR. we have cherry-picked it with a few other changes in 82bd4f0