Phase 24: add mix buses, coherent peak meters and master clipping indicators

docs/PROJECT_FORMAT.md

@@ -1,4 +1,4 @@
-# Project format v8 (reads v1–v7)
+# Project format v9 (reads v1–v8)
 
 Phases 5–15 use a bounded text format implemented in C99 without a parser library.
 The extension is `.xtc`. File operations run exclusively on the control thread.
@@ -192,7 +192,7 @@ or a claim of power-cut qualification.
 
 ## Phase 22: version 8 audio clip settings
 
-Current writer emits v8 and reads v1–v7 with unity clip gain and no explicit
+Phase 22 introduced v8 and reads v1–v7 with unity clip gain and no explicit
 clip fades. After **every** clip's `MIDI` record (including silent and MIDI
 clips), v8 requires:
 
@@ -215,3 +215,15 @@ a fresh ID/source offset, retains outer explicit fades and clears inner ones.
 No waveform cache or source audio is serialized. Checked-in v1–v7 migrations,
 v8 exact roundtrips, malformed new metadata and byte-mutation tests cover the
 format change (`make test test-portability test-audio-editing`).
+
+## Phase 24: version 9 mix routing
+
+The writer emits v9. Immediately after each TRACK's MIX record it writes
+`ROUTE is_bus bus_id`. `is_bus` is 0 or 1; `bus_id` is an existing bus track ID,
+or decimal UINT64_MAX (18446744073709551615) for master. Buses must route to
+master and cannot contain clips. Invalid destinations, flags and nested routing
+are rejected. v1–v8 migrate all tracks to ordinary direct-to-master tracks.
+Bus controls and automation use existing MIX and POINT records. Older readers
+reject v9 rather than discard routing. The public `xtc_track_info` grew fields;
+API consumers must rebuild. Fixtures cover v1–v8 migration and tests exercise
+v9 exact roundtrips and malformed routing.