[JasMehta08]

What

Adds docs/design/test-strategy.md:

• test levels (unit, integration on the Compose cluster, end-to-end on AWS)
• required tests per component, each with a TS-... ID
• failure tests, test data and what runs in CI

Closes #31

Why

Week 3 build issues (#19) need to know which tests they must ship with. Each build issue can now link the TS IDs it has to meet. The approach mirrors how Dropbox tests Magic Pocket: failure injection, deliberately corrupting data to test the verifiers, and zone drills [B3][T9].

How to check it

• Section 3: one table per component. Compare against the components in the brief and the exercise plan ([Design] Finalize the exercise plan, including the erasure coding scheme and parameters #14).
• The replica count, EC parameters, repair threshold, empty-block rule and zone tolerance come from section 8 of the exercise plan (docs(design): finalize the exercise plan with RS(6,3) on 9 nodes #56), so this PR should merge after docs(design): finalize the exercise plan with RS(6,3) on 9 nodes #56. The workload model reference needs docs(research): add the case workload model #47 first.
• Section 6: what runs on every PR vs before experiments.

Checklist

• cargo fmt, cargo clippy and cargo test pass locally (code PRs)
• Tests added or updated for the change (code PRs)
• New sources are added to docs/sources/README.md (research PRs)
• Design decisions are recorded in the Design Decision Log
• Docs and README are updated if behaviour or setup changed

[frectrium]

@JasMehta08 the content looks good to me, but a few parts depend on PRs that aren't merged yet. Can you update them so this doesn't land with placeholders?

1. The "(set by [Design] Finalize the exercise plan, including the erasure coding scheme and parameters #14)" placeholders. The exercise plan is up in docs(design): finalize the exercise plan with RS(6,3) on 9 nodes #56. Section 8 there has the values: 3 replicas, one per zone. RS(6,3), so k = 6 and m = 3. Repair threshold 30 s in tests. Empty blocks rejected with 400. Zone loss is tolerated, so TS-FAIL-5 always applies.
2. Service count. docs(design): finalize the exercise plan with RS(6,3) on 9 nodes #56 grows the Compose cluster to 9 storage nodes (3 per zone), so "six services" in section 2 and TS-SVC-1 becomes 12. Easiest is to say "every service in docker-compose.yml" so it doesn't go stale again. Same for [Infra] Integration test harness on the local cluster, run in CI #32.
3. "2 MB is the workload model's average ([Cost] Build the case workload model from Dropbox's public disclosures #15)" points at docs(research): add the case workload model #47, which isn't merged. Either merge docs(research): add the case workload model #47 first or drop the reference for now.

Suggested order: #47, then #55 and #56, then this one with the numbers filled in.