Initial commit: Go SRE interview exercises

Includes solved 02-worker-pool and 09-safe-counter (mutex, atomic, sharded).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GbnJj9ZqEx9nUvzkjdwfMq

README.md

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+# Go SRE Interview Gym
+
+A monorepo of hands-on exercises covering what actually gets asked in Go
+live-coding rounds for SRE roles. Each numbered folder is self-contained:
+
+- `main.go` — the assignment (in comments) + a scaffold with TODOs and a small
+  runnable demo
+- `main_test.go` — the interviewer: tests that define "correct", including the
+  traps (goroutine leaks, races, boundary conditions)
+- `README.md` — the deep dive: why the idiom exists, what breaks in production,
+  and the follow-up questions interviewers reach for
+
+## Setup
+
+Go ≥ 1.22 required (loop-variable semantics), any recent toolchain is fine —
+built and verified against **go 1.27**. Pure stdlib, zero dependencies.
+
+## Workflow
+
+```bash
+# pick an exercise, read the assignment in main.go, implement the TODOs
+go test -race ./01-url-health-checker/     # -race ALWAYS. non-negotiable.
+
+# run everything you've solved so far
+go test -race ./...
+
+# some exercises have benchmarks worth exploring
+go test -bench=. -cpu=1,4,8 ./09-safe-counter/
+```
+
+Interview-realistic rules: timer set to 30 min, no autocomplete, no LSP if
+you're brave, and **narrate your reasoning out loud** — the talking is graded
+as much as the code.
+
+## The ladder
+
+| # | Exercise | Core skill | Freq in interviews |
+|---|----------|-----------|--------------------|
+| 01 | url-health-checker | bounded concurrency, ctx, HTTP hygiene | ★★★★★ |
+| 02 | worker-pool | channel closing protocol, WaitGroup | ★★★★★ |
+| 03 | rate-limiter | lazy state, mutex, monotonic time | ★★★★☆ |
+| 04 | log-topn | streaming, Scanner limits, map→sort | ★★★★☆ |
+| 05 | lru-cache | map+list composite, O(1) design | ★★★★☆ |
+| 06 | retry-backoff | jitter, ctx-aware sleep, error classes | ★★★☆☆ |
+| 07 | fan-in | leak-free cancellation, nil-channel trick | ★★★☆☆ |
+| 08 | graceful-shutdown | signals, Shutdown ctx inversion, k8s | ★★★☆☆ |
+| 09 | safe-counter | mutex vs atomic vs sharding, -race | ★★★★☆ (as warm-up) |
+| 10 | gotchas | "what does this print" trivia round | ★★★★★ |
+
+Suggested order: 09 → 02 → 01 → 03 → 05 → 04 → 06 → 07 → 08, with gotchas (10)
+sprinkled daily. 09 first because it calibrates you on `-race`; 01/02 are the
+highest-yield; 08 is the best *conversation* generator in on-sites.
+
+## Ground rules baked into the tests
+
+- Tests use timeouts to catch deadlocks and missing `close()` — a hanging test
+  is a failing test with extra steps.
+- Several tests count goroutines or track peak concurrency with atomics —
+  read them after solving; test-writing techniques are interview material too.
+- Everything must pass under `-race`. A green test that fails `-race` is red.
+
+## Note on the gotchas folder
+
+`10-gotchas/*` are predict-then-run programs: ex02 deadlocks on purpose, ex11
+doesn't compile on purpose. Run them individually (`go run ./10-gotchas/ex01-loopvar/`),
+not as part of `go build ./...`. Answers are collapsed in `10-gotchas/README.md`.