docs(system-design): add coordination and consensus guide

[shalshcode08]

Summary\n\n- add a standalone, language-neutral Topic 8 guide to coordination, consensus, Raft, leader election, leases, fencing tokens, distributed locks, semaphores, barriers, and coordination services\n- include detailed leader-elected scheduler and fenced inventory allocator designs with failure timelines and resource-side correctness rules\n- cover production architecture, membership changes, snapshots, observability, security, performance, failure recovery, testing, exercises, interview Q&A, cheat sheet, and primary official references\n- register the guide in the System Design and Distributed Systems navigation manifest\n\n## Scope\n\nThis pull request changes only:\n\n- manifest.json\n- src/content/systemdesign/coordination-consensus-leases-locks.html\n\nNo tests, styling, shared design files, or TOPICS.md statuses were changed.\n\n## Research\n\nPrimary sources include the extended Raft paper and USENIX publication, the official Raft resource site, Paxos Made Simple from Microsoft Research, official etcd API guarantees and concurrency APIs, Apache ZooKeeper and Curator recipes, Kubernetes Lease and coordinated leader election documentation, and PostgreSQL locking and uniqueness documentation.\n\n## Validation\n\n- npx prettier --check manifest.json src/content/systemdesign/coordination-consensus-leases-locks.html\n- npm test\n- npm run build\n- git diff --check\n- scope check: exactly one new topic file plus manifest.json\n- footer check: exact July 2026 knowledge-base footer present once\n- no em dash found in new topic or manifest entry\n\n## Notes\n\nThe documentation remains framework-neutral, uses pseudocode first for algorithms, and uses a small TypeScript example only to demonstrate cancellation and fencing boundaries. TOPICS.md remains Pending until this pull request is merged.