refactor(api): typed enums for emoji-list fields (DiceEmoji, ReactionEmoji)

Two API fields carry restricted emoji-value sets that the scraper's
curly-quote regex strips during IR extraction (multi-byte boundary
issue): ReactionTypeEmoji.Emoji and sendDice.Emoji. They previously
typed as plain string with no compile-time guarantee on values.

Add hand-curated typed-string enums in api/enums.go (the manual file,
not enums.gen.go):

  - DiceEmoji: 6 constants (Dice, Dart, Basketball, Football, Bowling,
    SlotMachine) covering Telegram's full set for sendDice.
  - ReactionEmoji: 73 constants covering the canonical reaction set
    from https://core.telegram.org/bots/api#reactiontypeemoji. Names
    follow Unicode CLDR short names where one exists, otherwise stable
    common-English labels (e.g. ThumbsUp, Heart, Clown, ManTechnologist).

Wire the field-type override via cmd/genapi/emitter.go:

  - fieldTypeOverrides map keyed "<TypeOrParamsName>.<FieldName>".
  - goField/multipartFieldEntry consult the override after the enum-plan
    lookup; falls through to the default goType when nothing matches.
  - methods.tmpl gains goFieldP/multipartFieldEntryP helpers that pass
    the params type name as override-parent (the params struct doesn't
    share a Go type with the field, so the existing parent="" enum-key
    convention is preserved).

Regenerated api/types.gen.go and api/methods.gen.go now type the two
fields as ReactionEmoji and DiceEmoji respectively. No other Emoji
field is affected (override is scoped per parent type). regen-from-
fixture is byte-deterministic across runs.

Add api/emoji_enums_test.go covering const wire values, reflection
checks on field types, and a marshal/unmarshal round-trip for
ReactionTypeEmoji.

cmd/genapi/methods.tmpl

@@ -15,12 +15,12 @@ import (
 var _ = strconv.Itoa  // keep import for multipart helpers
 var _ = json.Marshal  // keep import for complex multipart fields
 
-{{range .Methods}}
+{{range .Methods}}{{$methodName := .Name}}
 // {{title .Name}}Params is the parameter set for {{title .Name}}.
 //
 {{docComment .Doc -}}
 type {{title .Name}}Params struct {
-{{range .Params}}{{docComment .Doc}}	{{goField "" .}}
+{{range .Params}}{{docComment .Doc}}	{{goFieldP $methodName .}}
 {{end}}}
 {{if .HasFiles}}
 // HasFile reports whether a multipart upload is required.
@@ -31,7 +31,7 @@ func (p *{{title .Name}}Params) HasFile() bool {
 // MultipartFields returns the non-file fields used in the multipart body.
 func (p *{{title .Name}}Params) MultipartFields() map[string]string {
 	out := map[string]string{}
-{{range .Params}}{{if not (isFileField .)}}{{multipartFieldEntry "" .}}{{end}}{{end}}	return out
+{{range .Params}}{{if not (isFileField .)}}{{multipartFieldEntryP $methodName .}}{{end}}{{end}}	return out
 }
 
 // MultipartFiles returns the file parts.