1.0.0-alpha5: Server-Timing replaces cookie cache-state stamping

Field testing alpha4 on an LSCache-fronted site exposed that
the Set-Cookie + absolute-expires pattern is fundamentally
incompatible with response caching. Alpha5 swaps to a
Server-Timing header, which has no expiry mechanism and so
survives edge caching cleanly.

See ROADMAP.md for full details.