Add 5 deep dives, 64 company guides, and expand the flashcards

[design-gurus]

Three batches: the missing landmark systems, the company index expansion, and flashcard coverage for everything added recently.

Deep dives (14 to 19)

• Borg and Kubernetes: the biggest gap. Declarative desired state plus the reconciliation loop, bin packing and preemption, resource reclamation, and why the control plane becomes the most critical system you run.
• Redis internals: single-threaded command execution as the source of atomicity, RDB vs AOF, Sentinel and Cluster, and the honest cost (one slow command blocks every client).
• Elasticsearch and Lucene: inverted index, analysis chain, BM25, immutable segments, and the distributed layer where deep pagination is expensive and scoring is approximate.
• Flink: event time vs processing time, watermarks, distributed snapshots, and exactly-once explained honestly as a property of the whole pipeline rather than a checkbox.
• HNSW and vector databases: the graph walk, the recall-versus-latency dial, and filtered search as the genuinely hard part. Ties into the AI positioning and pairs with the Lucene page (keyword and vector retrieval are complements).

Companies (58 to 122)

64 new guides covering every company that now has a published DesignGurus system-design answer, including Duolingo (which was waiting on the answers campaign), Lyft, Snap, Twitch, Epic Games, Instacart, eBay, Booking.com, GitLab, Docker, HashiCorp, Temporal, Supabase, Okta, Snyk, Elastic, Plaid, Brex, Klarna, Revolut, Goldman Sachs, JPMorgan Chase, Hudson River Trading, Jump Trading, Zoox, Neuralink, Rivian, Boston Dynamics, IBM, SAP, Cisco, Zoom, Dropbox, and Slack.

Each page is distilled from the guide already published on designgurus.io, so the two properties agree rather than drifting, and each links back to its full company guide. Every page cross-links patterns and practice questions from this repo, including the newly added ones.

Built through the existing companies pipeline. The 58 existing pages regenerate byte-identical, so the diff is purely additive.

Flashcards (79 to 110 cards)

Coverage for the six patterns, ten questions, five cheat sheets, and five deep dives added recently. Three new sections: distributed transactions and event-driven state (2PC, sagas, the dual-write problem, outbox, event sourcing), landmark systems, and recurring design problem shapes (identified vs counted inventory, holds without distributed locks, spatial indexes, SFU topology, prefix caching, token-budget quotas). Cards for logical clocks, gossip, and backpressure policies land in their existing sections.

Checks

Repo link checker passes across all 70 new files. No em dashes, per CONTRIBUTING. No year references. Every company page's answer link was verified live before generation; one slug the writer guessed returned 404 and was corrected to the published slug.