Prune shipped items from the roadmap and renumber what is left

[chrisuthe]

docs/ROADMAP.md goes from 28 items (2694 lines) to 15 open ones. Git history keeps the record of how each item shipped.

What changed

• Remove the What the scaffold already does section and every item that is fully shipped.
• Cut partly-shipped items down to what is still owed. The gaps that were buried inside items marked shipped are kept as items of their own:
  • state-file debounce
  • signing/notarization and a minimum-macOS gate
  • the four unit-hardening directives
  • sink-contract, mdns_dnssd and TSan coverage
  • Intel-Mac build and glibc floor
  • conf.example and wiki version drift guards
  • PortAudio/ALSA recovery hardware passes
  • PipeWire ring resize and the -Wrestrict exemptions
  • refusing userinfo in -s
  • multi-group switch
• Group the library-side gaps into one item: no log sink hook, a taken --port the CLI cannot see, and no connection origin.
• Renumber the items 1–15 and update the references to them:
  • docs/wiki/Getting-Started-on-a-Raspberry-Pi.md → item 5
  • docs/wiki/Home.md → item 10
  • docs/wiki/Installation.md → item 7
  • docs/wiki/Running-as-a-Service.md → item 8
  • the .pkg welcome pane in scripts/build_macos_pkg.sh → item 7
• Change the roadmap link text in the release.yml notes and Home.md to "what is still owed".

Re-checked against sendspin-cpp v0.9.5 (the current pin)

• Old item 17 (activities-based admission) is removed as done. v0.9.5 implements activities, server/activate and PSK pairing (src/admission.h, connection_manager.cpp), and the CLI already exposes --pairing-code and --allow-unpaired.
• --interface (now item 15) is still blocked upstream. src/host/ws_server.cpp:61 still binds "0.0.0.0", and SendspinClientConfig has no bind-address field. Its "no inbound authentication" premise no longer holds: pairing and admission gate the inbound path now. The item is rewritten to say so; what is left is narrowing exposure on multi-homed hosts and interface-scoped advertisement.
• The required_lead_time_ms/min_buffer_ms gap is removed as closed upstream. v0.9.5 fills both in player_role.cpp.
• No log sink hook yet. SS_LOG* is still bare fprintf, and the dialled URL is still logged unmasked at info/error.

Compatibility

Nothing breaks for CLI users. This changes docs, one string in the macOS installer's welcome pane, and one line of release-notes text. Flags, config keys, the control-socket protocol, the state format, exit codes and runtime behaviour are all unchanged.

Left as-is: src/main.cpp:583 / docs/wiki/Troubleshooting.md:196 ("See docs/ROADMAP.md" for a no-mDNS build) and src/cli.cpp:1126 still point at the roadmap generally. Rewording them would change log and help strings, which is outside this PR.