Cache coalesce lab

Single-flight style request coalescing.

Package

Local module path (relative to the gocrazy app):

labs/cachecoalesce

Run from the labs module root:

cd websites/gocrazy.dev/labs
go test ./...

Or via:

./all-infra/scripts/kt.sh gocrazy test-labs

Goal

Read the package, run the tests, then break one invariant on purpose and watch a test fail — that failure is the lesson.