essessey v0.6.1 — per-binding docs, error context

Each binding now has a README written for someone who does not already know how
this works, with runnable examples: sse (handler to bytes to parsing), nats
(subject scheme, and the fact that event ids are not carried there), ws (the
one-concurrent-writer rule).

The sse README leads with the gap this package cannot close in code -- it does
not set Content-Type: text/event-stream, and a browser fails the connection
without it.

The root README gained the read-side example it never had: Reassemble is half of
what this library does and had no example anywhere.

Ten call sites now wrap their errors with ctxerrors instead of returning them
bare, and the root layout block finally lists store.go and multisink.go.

No API change, no behaviour change.