The translate card, pointed the other way, and a call that no longer happens

Phase 28's step (b): both remaining items are about direction, and both had
a wrong answer that looked right.

isTranslation could not simply be read backwards. readsAsEnglish is a
deliberately low bar — Latin letters, not swamped by another script — which
every Portuguese sentence clears as easily as English does, so swapping its
two halves would have called every genuine Portuguese correction inside a
Portuguese document a translation. The flipped direction uses sentenceLang
from doclang.go instead, where English has its own curated marker list and
has to out-evidence the pair language to win. The English-document path is
untouched; reconcilePending carries the verdict to ask the question the
right way round.

The tap-through's whole observable change is a model call that stops
happening. /suggestions/{id}/translate now recovers the explanation's
language by re-running targetFor rather than assuming the pair, which gives
today's answer everywhere except the case that was broken: the Portuguese
writer whose explanation already arrived in Portuguese, previously
round-tripped through the model into Portuguese again. It answers "" there,
and the client's existing `res.translation.trim() || explanation` fallback
seeds the bubble with the explanation itself — no frontend change at all.
It deliberately does not render that explanation into English on the
grounds that English is technically the other half: an unasked-for
rendering into the language she is practising is noise, not a seed.

Tests pin both directions of the detector, with Portuguese-in-Portuguese as
the case the file exists for, plus four handler tests through the real
/check and /translate paths — including the skipped seed asserting the
model was never called, and the learning_pair zh learner whose English
explanation still renders into Chinese.

Left of the phase: (c) the garden's language tagging and read-aloud.

Claude-Session: https://claude.ai/code/session_01GJHNvirh7Hzhc9RL3HAvz7

internal/suggestions/doclang_test.go

@@ -1,6 +1,7 @@
 package suggestions
 
 import (
+	"encoding/json"
 	"net/http"
 	"path/filepath"
 	"strings"
@@ -259,3 +260,136 @@ func TestLanguageFlipReopensCheckedSentences(t *testing.T) {
 		t.Fatalf("the already-checked sentence was not re-opened by the flip:\n%s", client.lastPrompt)
 	}
 }
+
+// The translate card, pointed the other way. She is writing her journal in
+// Portuguese and drops in the one English sentence she knows; Petal renders it
+// into Portuguese, and that card is a translation — not a correction to prose
+// that was never wrong.
+func TestEnglishSpanBecomesATranslateCardInAPortugueseDocument(t *testing.T) {
+	const english = "I want to say this but I don't know how to say it."
+	// The model volunteers "clarity", as it did for the zh case. Not consulted.
+	client := &stubClient{response: `{"suggestions":[
+		{"original":"` + english + `","replacement":"Eu quero dizer isto mas não sei como o dizer.","explanation":"Aqui está em português.","type":"clarity"}
+	]}`}
+	srv, docID, _ := newDirectedServer(t, client, "pt-PT", auth.DirectionLearningEn, ptDocument+" "+english)
+
+	var out []db.Suggestion
+	rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("check: code=%d body=%s", rec.Code, rec.Body)
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(out) != 1 {
+		t.Fatalf("want 1 card, got %d: %+v", len(out), out)
+	}
+	if out[0].Type != db.SuggestionTypeTranslate {
+		t.Fatalf("card type = %q, want %q", out[0].Type, db.SuggestionTypeTranslate)
+	}
+}
+
+// And the half that keeps it honest: a genuine Portuguese correction in the same
+// document stays a correction. Reading the English-document test backwards would
+// have called this a translation, because every Portuguese sentence also "reads
+// as English" by that test's deliberately low bar.
+func TestPortugueseCorrectionKeepsItsTypeInAPortugueseDocument(t *testing.T) {
+	client := &stubClient{response: `{"suggestions":[
+		{"original":"Não sei porque isso é tão difícil para mim.","replacement":"Não sei porque isto é tão difícil para mim.","explanation":"Aqui usa-se isto.","type":"grammar"}
+	]}`}
+	srv, docID, _ := newDirectedServer(t, client, "pt-PT", auth.DirectionLearningEn, ptDocument)
+
+	var out []db.Suggestion
+	rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("check: code=%d body=%s", rec.Code, rec.Body)
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(out) != 1 {
+		t.Fatalf("want 1 card, got %d: %+v", len(out), out)
+	}
+	if out[0].Type == db.SuggestionTypeTranslate {
+		t.Fatal("a Portuguese correction inside a Portuguese document was labelled a translation")
+	}
+}
+
+// setDocLang writes a document's language verdict directly, so a test of the
+// tap-through doesn't have to run a checkpoint through the same stub client to
+// get one.
+func setDocLang(t *testing.T, h *Handler, docID, lang string) {
+	t.Helper()
+	if _, err := h.DB.Exec(`UPDATE documents SET doc_lang = ? WHERE id = ?`, lang, docID); err != nil {
+		t.Fatalf("set doc_lang: %v", err)
+	}
+}
+
+// seedExplanation files one card carrying a given explanation and returns its
+// id — the shape the translate tap-through needs, where only the explanation and
+// the document it hangs off matter.
+func seedExplanation(t *testing.T, h *Handler, docID, explanation string) string {
+	t.Helper()
+	var sugID string
+	if err := h.DB.QueryRow(
+		`INSERT INTO suggestions (doc_id, original, replacement, explanation, type, from_pos, to_pos)
+		 VALUES (?, ?, ?, ?, ?, 0, 5) RETURNING id`,
+		docID, "isso", "isto", explanation, "grammar",
+	).Scan(&sugID); err != nil {
+		t.Fatalf("seed suggestion: %v", err)
+	}
+	return sugID
+}
+
+// The tap-through has to read the same decision the card was written under. On a
+// Portuguese document by a Portuguese writer the explanation already arrived in
+// Portuguese, and the old endpoint would have sent it to the model to be
+// rendered into Portuguese again.
+func TestTranslateSkipsWhenTheExplanationIsAlreadyHers(t *testing.T) {
+	client := &stubClient{response: "Não devia ser chamado."}
+	srv, docID, h := newDirectedServer(t, client, "pt-PT", auth.DirectionLearningEn, ptDocument)
+	setDocLang(t, h, docID, docLangPair)
+	sugID := seedExplanation(t, h, docID, "Aqui usa-se isto.")
+
+	rec := do(t, srv, http.MethodPost, "/suggestions/"+sugID+"/translate", "")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("translate: code=%d body=%s", rec.Code, rec.Body)
+	}
+	var out translateResponse
+	if err := json.Unmarshal(rec.Body.Bytes(), &out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if out.Translation != "" {
+		t.Fatalf("translation = %q, want empty: the bubble seeds from the explanation itself", out.Translation)
+	}
+	if client.calls != 0 {
+		t.Fatal("the model was asked to render Portuguese into Portuguese")
+	}
+}
+
+// The learner travelling the other way is the case that proves the endpoint
+// derives its destination rather than skipping whenever a document is flipped: a
+// native English speaker learning Chinese, writing Chinese, gets her
+// explanations in English — and the tap still has somewhere to go.
+func TestTranslateStillRendersForALearnersEnglishExplanation(t *testing.T) {
+	const zhDocument = "今天天气很好。我早上去公园散步。下午我在家里写作业。晚上我和朋友一起吃饭。"
+	client := &stubClient{response: "这里应该用这个。"}
+	srv, docID, h := newDirectedServer(t, client, "zh", auth.DirectionLearningPair, zhDocument)
+	setDocLang(t, h, docID, docLangPair)
+	sugID := seedExplanation(t, h, docID, "This measure word doesn't fit here.")
+
+	rec := do(t, srv, http.MethodPost, "/suggestions/"+sugID+"/translate", "")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("translate: code=%d body=%s", rec.Code, rec.Body)
+	}
+	var out translateResponse
+	if err := json.Unmarshal(rec.Body.Bytes(), &out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if out.Translation == "" {
+		t.Fatal("a learner's English explanation was left untranslated")
+	}
+	if !strings.Contains(client.lastPrompt, "Simplified Chinese") {
+		t.Fatalf("translate didn't render into the pair language:\n%s", client.lastPrompt)
+	}
+}