perf(dispatch): typed Context.Command/CommandArgs/RegexMatch fields

Move the three conventional Values keys ("command", "command_args", "regex_match") to typed fields on Context. Router and group routing write the fields directly; the Values map is allocated lazily via the new Set method and reserved for user-defined custom keys.

Allocation impact (M4 Max, b.Loop()):

  DispatchCommand:   5 allocs/op -> 1, 153ns -> 69ns (-55%)

  DispatchTextRegex: 5 allocs/op -> 2, 181ns -> 107ns (-41%)

  DispatchFilter:    2 allocs/op -> 1, 32ns -> 19ns (-41%)

  NewContext:        5.79ns -> 1.60ns

Trade-off: Context struct grew from ~48B to ~96B (three new fields), so filter-only paths pay ~50B more per dispatch. Command/regex paths save ~320B + 4 allocs each, which dominates for typical bot workloads.

Handlers reading c.Values["command"], c.Values["command_args"], or c.Values["regex_match"] now get nil; the typed fields c.Command, c.CommandArgs, c.RegexMatch are the new accessors. Custom keys still work via c.Set(k, v) and c.Values[k].

dispatch/groups.go

@@ -138,8 +138,8 @@ func (r *Router) runGroupHandlers(c *Context, m *api.Message, g int) (matched bo
 			if route.group != g || route.cmd != cmd {
 				continue
 			}
-			c.Values["command"] = cmd
-			c.Values["command_args"] = args
+			c.Command = cmd
+			c.CommandArgs = args
 			if err := route.handler(c, m); err != nil {
 				if errors.Is(err, ErrContinueGroups) {
 					continue
@@ -159,7 +159,7 @@ func (r *Router) runGroupHandlers(c *Context, m *api.Message, g int) (matched bo
 			if subs == nil {
 				continue
 			}
-			c.Values["regex_match"] = subs
+			c.RegexMatch = subs
 			if err := route.handler(c, m); err != nil {
 				if errors.Is(err, ErrContinueGroups) {
 					continue