docs(api): document whisper language selection contract surface (T8.7, M4.2)

docs/04-api-contracts.md

@@ -15,7 +15,11 @@ each command returns `Result<T, WaError>` where `WaError` carries a `kind` (mach
 // `record` (default false) controls audio RETENTION (ADR-0009). When false, working audio is
 // deleted on finalize and only the transcript/notes persist. It can be toggled mid-meeting.
 // templateId (Phase 8, T8.1, FR-NOTE-5) picks a NoteTemplate — see list_note_templates below.
-start_recording(input: { meetingTitle?: string; calendarEventId?: string; record?: boolean; templateId?: string }): MeetingId
+// language (T8.7, FR-TRX-4, M4.2): omitted/"auto" requests auto-detection; an ISO-639-1 code
+// (e.g. "es") forces that language. Falls back to Settings.whisper_language when omitted.
+// Only takes effect with a multilingual model loaded (ModelInfo.multilingual) — an English-only
+// model forces "en" regardless (see resolve_language in src-tauri/src/transcription/mod.rs).
+start_recording(input: { meetingTitle?: string; calendarEventId?: string; record?: boolean; templateId?: string; language?: string }): MeetingId
 stop_recording(input: { meetingId: MeetingId }): MeetingSummaryRef
 pause_recording(input: { meetingId: MeetingId }): void
 resume_recording(input: { meetingId: MeetingId }): void
@@ -27,11 +31,17 @@ hardware_status(): { backends: BackendInfo[]; active: BackendId; modelSize: stri
 set_preferred_backend(input: { backend: BackendId | "auto" }): void
 
 // ---- Transcription / models ----
-reprocess_transcript(input: { meetingId: MeetingId; model: string }): void   // batch mode (FR-TRX-3)
+// language (T8.7, M4.2): omitted reuses the meeting's current language rather than resetting it.
+reprocess_transcript(input: { meetingId: MeetingId; model: string; language?: string }): void   // batch mode (FR-TRX-3)
+// ModelInfo gained `multilingual: boolean` (T8.7, FR-TRX-4, M4.2) — false for `.en` (English-only)
+// ggml variants, true for the multilingual ones; gates the Settings language picker.
 list_models(): ModelInfo[]
 list_diarization_models(): ModelInfo[]        // fixed seg+emb pair (T4.7, FR-MODEL-1)
 download_model(input: { kind: "whisper" | "diar-seg" | "diar-emb"; id: string }): void  // emits progress events
 remove_model(input: { id: string }): void     // disambiguated by id, not kind — ids never collide across catalogs
+// Static catalog of whisper.cpp-recognized ISO-639-1 codes for the Settings language dropdown
+// (T8.7, FR-TRX-4, M4.2); "Auto-detect" is a frontend-only addition, not in this list.
+list_whisper_languages(): { code: string; label: string }[]
 
 // ---- Speakers ----
 rename_speaker(input: { meetingId: MeetingId; label: string; name: string }): void
@@ -106,6 +116,9 @@ run_agent(input: { briefId: string; tool: "claude" | "codex" | "opencode" | "cop
 create_issue_from_brief(input: { briefId: string; tracker: "github"; assignCopilot?: boolean }): { url: string }            // FR-AGENT-2
 
 // ---- Settings ----
+// Settings gained `whisper_language: string | null` (T8.7, FR-TRX-4, M4.2) — the default
+// transcription language applied at the next start_recording; null = auto-detect. Mirrors
+// this doc's settings.json `transcription.language` (03-data-model.md).
 get_settings(): Settings
 update_settings(input: Partial<Settings>): Settings
 // Reports the full egress allowlist so the UI can prove exactly what may leave the device (FR-SEC-2).