Cut v0.2.0: version bump, release notes, roadmap

[chrisuthe]

Summary

Cuts the v0.2.0 release:

• Bump CMakeLists.txt to VERSION 0.2.0 (matches the v0.2.0 tag the release workflow will publish).
• Add docs/release-notes/0.2.0.md — the body release.yml publishes. It leads with the breaking -s change, then the --audio-format list, the sink playback-clock and AF_NETLINK fixes, and the under-the-hood work, and ends with the v0.1.6...@TAG@ compare link.
• Refresh the roadmap's "What the scaffold already does" summary so -s reads as discovery-only; the stale line still said it dials a literal address, which Remove literal-address -s dialling; keep only -s mdns: discovery #56 removed. Item 5 and the new item 26 stay as the historical record.

The wiki needs no change here: #56 and #57 already brought the -s and --audio-format pages current, so wiki.yml re-mirrors docs/wiki/ unchanged on merge.

What ships in v0.2.0 (since v0.1.6)

• Breaking: literal -s <address> dialling removed; -s mdns: discovery only (Remove literal-address -s dialling; keep only -s mdns: discovery #56).
• --audio-format takes an ordered list of preferred formats (Let --audio-format take an ordered list of preferred formats #57).
• A recovered audio device no longer jumps the playback clock — fixed for ALSA (ALSA: after a replugged USB DAC is recovered, playback stalls in a loop of underruns until the player restarts #54, Fix ALSA playback clock after device recovery #55) and extended to the Pulse/PortAudio/PipeWire sinks (Pulse, PortAudio and PipeWire sinks leave outage frames in the playback clock #58, Retire the outage gap in the Pulse, PortAudio and PipeWire sinks #59).
• Allow AF_NETLINK in the systemd unit so the MAC-derived client id is detected (Allow AF_NETLINK in the systemd unit so the MAC-derived client id is detected #53).
• Re-pin sendspin-cpp to v0.8.0 and adopt start()/stop() (Re-pin sendspin-cpp to v0.8.0 and adopt its start/stop lifecycle #62).
• clang-format CI gate + scripts/format.sh (Make the tree clang-format clean and gate it in CI #61); comment guidelines in AGENTS.md (Trim codebase comments to one-line, caller-facing style #60).
• README restructured around what / how-to-run / how-to-contribute (Restructure README around what/how-to-run/how-to-contribute #52); ARMv6 leg moved to its own workflow (Move the ARMv6 leg out of core CI into a workflow of its own #51).

Compatibility

This release is usage-breaking for anyone upgrading from v0.1.6. #56 removed literal-address -s dialling: a typed-in host, host:port, or ws:// URL — on the command line or as server = in a config file — is now a parse error. -s mdns:[<name>] (discovery) is the only form left. An install still configured with an address fails loudly at startup rather than quietly changing how it connects; point the server at this player, or switch to -s mdns:.

The changes in this PR — a version-string bump and documentation — move no control surface themselves; they cut a release around #56, which is already on main.

Other visible-but-non-breaking changes bundled in the release: the sink playback-clock fixes (#55, #59, bug fixes), --audio-format now accepts an ordered preference list (#57, additive), the AF_NETLINK systemd fix (#53, packaging), and the re-pin of sendspin-cpp to v0.8.0 (#62) — a dependency bump that can shift runtime behaviour without touching a flag, assessed in its own PR.

Cutting the tag

After this merges to main and CI is green, pushing v0.2.0 runs release.yml (preflight validated locally: single VERSION 0.2.0 line, notes exist with balanced fences and ###-or-deeper headings) → the build legs → a published sendspin-cli 0.2.0 release with the five *.tar.gz archives, the macos-arm64.pkg, and SHA256SUMS.