docs(examples): split cmd/quickstart into Quick Start + e2e variants

[F2077]

Why

README's Quick Start is a 45-line inline snippet, but the runnable
program in cmd/quickstart had been growing into a 13-phase
~260-line end-to-end walk-through. The two pieces drift
independently:

• README was using outdated ceremony (panic instead of log.Fatal,
  IIFE-wrapped defer Close, a 5-second WithTimeout that could
  never fire in a happy-path run, a // Output: "..." comment
  that's a godoc convention that doesn't apply to cmd/
  binaries).
• The program had been refined to cover every exported symbol in
  a single go run and to demonstrate the documented API
  contract on its own.

The fix: give each piece a single, clear job.

What this PR does

New cmd/quickstart (73 lines)

A minimum viable example that mirrors README's Quick Start 1:1.
One broker, one publisher, one subscriber, one topic, one Publish,
one Receive. Synchronous Publish means the 200 ms WithTimeout
demonstrably resets on the happy path (the select hits the Ch
branch, never the ErrCh one).

go run ./cmd/quickstart
# → Received: CPU over 90%!

cmd/quickstart-e2e (renamed, content unchanged)

The 13-phase end-to-end walk-through that previously lived at
cmd/quickstart/main.go. Same surface, same phase numbering, same
output. Only the trailing "quickstart: ok" was renamed to
"quickstart-e2e: ok" to match the new binary name.

go run ./cmd/quickstart-e2e
# → 30-line transcript ending in "quickstart-e2e: ok"

Docs

• README's Quick Start is rewritten to be 1:1 with the new
  simplified binary.
• cmd/quickstart/README.md is rewritten to describe the
  minimal binary and point at cmd/quickstart-e2e/README.md for
  the longer walk-through.
• cmd/quickstart-e2e/README.md is the previous contents of
  cmd/quickstart/README.md with internal references updated to
  the new path.
• CHANGELOG.md adds a ### Changed (examples split) section
  summarising the move.

What's not in this PR

• No library code changes. All pubsub/ code is identical to
  main. The existing unit tests in pubsub/ remain the contract.
• No CI changes. The existing .github/workflows/test.yml is
  the contract; both cmd/quickstart/main.go and
  cmd/quickstart-e2e/main.go already pass gofmt -l,
  go vet ./..., and go test -race -count=1 ./....

Verified locally

$ go run ./cmd/quickstart
Received: CPU over 90%!

$ go run ./cmd/quickstart-e2e
... (30-line transcript) ...
quickstart-e2e: ok

$ go build ./...               ok
$ go vet ./...                 clean
$ gofmt -l $(git ls-files)     clean
$ go test -race -count=1 ./... 3.1s, green