Change network protocol selections at runtime

Protocol selection previously belonged to tsng up and therefore to the
service command line. A blocked or degraded carrier could only be removed
by editing the service and restarting the whole agent. Make the ordered
selection a local per-network setting instead.

Add Agent::set_protocols and a local control request shared by CLI and tray.
Keep tsng protocols without arguments as the protocol catalog; naming a
network shows its selection and supplying a comma-separated list replaces
it. Accept exact names, full ids and unique prefixes using shared selector
rules; ambiguous selectors and invalid lists change nothing. An explicit
none selects control-only operation. Offline edits take the directory lock
and apply on the next start; live edits never write behind the daemon.

Register every built-in plugin and transport in the CLI. Treat up --protocol
as ordered initial defaults for networks without an override, allowing a
protocol absent from the unit to be enabled later. Retain the targeting of
unqualified startup options and allow explicitly scoped options to configure
initially disabled protocols. Persist NULL for inherited preferences and an
empty selection distinctly in SQLite schema 8; migration preserves existing
identities, secrets, signed records and counters.

Serialize changes with network lifecycle operations and commit preferences
before applying or announcing them. Retire only the affected network's data
links and E2E channels, cancel pending opens, and reject obsolete completion
results by attempt id. Clear peer data channels when authenticated protocol
capabilities change while retaining control sessions and topology. Replay
allocations and peer identities after asynchronously prepared plugins become
ready. A control-only start defers creation of the agent's single interface
until data is enabled; plugins receive a sink that can use it later. Automatic
carrier fallback continues to preserve E2E state; explicit policy changes
renegotiate it. Identity derivation and peer wire formats are unchanged.

Add a collapsible protocols row to each tray network tile, with a catalog
provided by the daemon, descriptions, immediately applied checkboxes and
preference-order buttons. Keep editing disabled until the worker refreshes
status so a second click cannot overwrite an edit using a stale snapshot.
Local IPC wire version is 20 for the new request and report fields; upgrade
and restart agent, CLI and tray together. Crate release versions remain unchanged; the requested RC tag is v0.1.0-rc.30.

Validation on Linux through nix develop:
- cargo fmt --all -- --check
- cargo clippy --locked --workspace --all-targets -- -D warnings
- cargo test --locked --workspace --all-targets: 509 passed, 2 existing ignored
  tests, with inherited TSUNAGI_CONTROL_SOCKET and TSUNAGI_CONTROL_GROUP removed

Tests use real iroh membership, SQLite, UDP, AWG and WireGuard encryption,
QUIC carriers and memory TUNs. Cover either endpoint-id role, full-size packet
delivery after switching, an unaffected second active network, saved selection
after restart, empty/invalid selections, first enablement without restart,
rapid changes racing old opens, live and offline CLI changes, startup-absent
protocols and preservation of signed state through migration. Earlier compile
and focused-test iterations caught initializer mistakes, a short test secret,
asynchronous plugin allocation readiness, and an inherited system socket
path; corrected before the successful final checks. Plain cargo was unavailable
in the host shell, so validation used the Nix toolchain. Manual tray interaction,
macOS/Windows, privileged host TUNs and public-network traversal were not run.

README.md

@@ -112,6 +112,23 @@ fresh masking profile and outer keys for each pair connection attempt, including
 reentry after fallback. Sustained data loss triggers switching independently of
 the iroh control connection. [Configuration and limits](docs/amnezia.md).
 
+Change a network's protocol selection without editing its service or restarting:
+
+```sh
+tsng protocols                         # available protocols and settings
+tsng protocols home                    # this device's choice for home
+tsng protocols home awg,wg-quic         # prefer AWG, retain QUIC as fallback
+tsng protocols home none                # control plane only
+```
+
+Exact network names, full ids and unique id prefixes work here. The choice is
+local to this device, saved per network and applied immediately; an offline
+change applies on the next start. In the tray, expand **protocols** on a network
+tile to toggle protocols or change their preference order. Both endpoints need
+a common protocol at the same wire version. `tsng up --protocol` supplies the
+initial defaults for networks without a saved choice; it does not restrict
+which built-in protocols can be enabled later.
+
 ## Platforms
 
 | Platform                  | Build        | CLI `tsng` | Tray | Exit node: offer / use | Packages                            |