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].

examples/callback/handlers.go

@@ -21,7 +21,7 @@ func handleStart(c *dispatch.Context, m *api.Message) error {
 }
 
 func handleCallback(c *dispatch.Context, q *api.CallbackQuery) error {
-	groups := c.Values["regex_match"].([]string)
+	groups := c.RegexMatch
 	current, _ := strconv.Atoi(groups[1])
 	if groups[2] == "inc" {
 		current++