feat(channels): add Channels.match() API and switch default to target mode

- Add Channels.match(name, ch) for dual-mode event filtering
  Returns true if ch matches any active channel for the logical name.
  In dual mode, matches both legacy and target channels.
- Switch default channelMode from 'current' to 'target'
  RT channels now resolve to 4210-4213 instead of legacy 100-103.
- Config default also updated so fresh installs start in target mode.
- Rollback to 'current' mode still supported via setMode().

apis/channels.lua

@@ -52,15 +52,15 @@ Channels.registry = {
 }
 
 --- Channel mode controls which channel number is returned by get().
--- "current" — use legacy channel numbers (default, safe)
--- "target"  — use new channel numbers (post-migration)
+-- "current" — use legacy channel numbers (for rollback)
+-- "target"  — use new channel numbers (default, post-migration)
 -- "dual"    — listeners open both; senders use target
-Channels.mode = 'current'
+Channels.mode = 'target'
 
 --- Load channel mode from persistent config if available.
 -- Called automatically on require; can be called again to reload.
 function Channels.loadConfig()
-  local cfg = Config.load('platform', { channelMode = 'current' })
+  local cfg = Config.load('platform', { channelMode = 'target' })
   if cfg.channelMode then
     Channels.mode = cfg.channelMode
   end