Add semver notes

crates/arti-client/semver.md

+ADDED: `channel` configuration, and `channnel.padding` config variable.

crates/tor-cell/semver.md

+ADDED: PaddingNegotiateCmd caret_int
+BREAKING: PaddingNegotiate constructors changed

crates/tor-chanmgr/semver.md

+BREAKING: get_or_launch takes a ChannelUsage argument
+BREAKING: ChanMgr::new takes new dormancy, config, and netparams arguments
+BREAKING: AbstractChannel has a new method engage_padding_activities
+ADDED: ChannelConfig, Dormancy, ChannelUsage

crates/tor-circmgr/semver.md

+BREAKING: Circuit construction now requires a ChannelUsage

crates/tor-config/semver.md

+ADDED: ReconfigureError::Bug enum variant
+ADDED: misc::PaddingLevel

crates/tor-proto/semver.md

 BREAKING: Remove key-specific accessors.
+BREAKING: ChannelsParamsUpdates renamed to ChannelPaddingInstructionsUpdates
+ADDED: Channel::engage_padding_activities
+ADDED: New methods for updating dormancy, netparams, and config
+BREAKING: padding::Parameters fields (builder methods) renamed to remove _ms suffix
+ADDED: padding::Parameters constructor disabled, and method padding_negotiate_cell
+BREAKING: ChannelsParamsUpdates::total_update changed, is now initial_update
+ADDED: ChannelPaddingInstructionsUpdates::combine, and field accessors