Add lossless M4A concatenation tool

Shell script to recombine individual story m4a files back into
per-cassette files using ffmpeg concat demuxer (stream copy, no
re-encoding). Generates chapter markers from input filenames and
preserves album art.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/sdlamp2-fsd.md

@@ -38,6 +38,13 @@ This document specifies the functional requirements for an SDL2 based media play
 
 ## 5. Changelog
 
+### 2026-02-11 — Lossless M4A concatenation tool
+
+- **New tool**: `tools/concat_cassette.sh` — losslessly concatenates multiple m4a files into a single file using the ffmpeg concat demuxer (`-c copy`, no re-encoding). Designed to recombine individual story files back into per-cassette files.
+- **Chapter markers**: Generates ffmpeg metadata with one chapter per input file, titled from the input filename (sans extension). Chapters enable navigation within the combined file.
+- **Album art preserved**: Attached pictures from the first input file carry through automatically via stream copy.
+- **Fast-start output**: Uses `-movflags +faststart` to place the moov atom at the front for better seeking.
+
 ### 2026-02-10 — Volume control and d-pad/keyboard navigation
 
 - **Cursor-based navigation**: Replaced mouse input with a focus-highlight model. Arrow keys (Left/Right) and d-pad move a visible blue highlight between UI elements; Enter/A button activates the focused button. Designed for use on a handheld gaming device with no mouse.

tools/concat_cassette.sh

@@ -0,0 +1,107 @@
+#!/usr/bin/env bash
+#
+# concat_cassette.sh — Losslessly concatenate m4a files into a single file
+# with chapter markers derived from input filenames.
+#
+# Usage:
+#   ./tools/concat_cassette.sh -o "Cassette 1.m4a" story1.m4a story2.m4a story3.m4a
+#
+# Requires: ffmpeg, ffprobe
+
+set -euo pipefail
+
+usage() {
+  echo "Usage: $0 -o OUTPUT INPUT1 INPUT2 [INPUT3 ...]"
+  echo ""
+  echo "Losslessly concatenate m4a files into a single file with chapter markers."
+  echo "Album art is preserved from the first input file."
+  echo ""
+  echo "Options:"
+  echo "  -o OUTPUT   Output filename (must not already exist)"
+  exit 1
+}
+
+die() {
+  echo "Error: $1" >&2
+  exit 1
+}
+
+# --- Parse arguments ---
+
+output=""
+while getopts "o:h" opt; do
+  case "$opt" in
+    o) output="$OPTARG" ;;
+    h) usage ;;
+    *) usage ;;
+  esac
+done
+shift $((OPTIND - 1))
+
+[ -z "$output" ] && die "Missing -o OUTPUT"
+[ $# -lt 2 ] && die "Need at least 2 input files"
+[ -e "$output" ] && die "Output file already exists: $output"
+
+for cmd in ffmpeg ffprobe; do
+  command -v "$cmd" >/dev/null 2>&1 || die "$cmd not found"
+done
+
+inputs=("$@")
+for f in "${inputs[@]}"; do
+  [ -f "$f" ] || die "Input file not found: $f"
+done
+
+# --- Set up temp dir ---
+
+tmpdir=$(mktemp -d)
+trap 'rm -rf "$tmpdir"' EXIT
+
+concat_list="$tmpdir/concat.txt"
+metadata="$tmpdir/metadata.txt"
+
+# --- Build concat list and probe durations ---
+
+echo ";FFMETADATA1" > "$metadata"
+
+cumulative_ms=0
+for f in "${inputs[@]}"; do
+  # Concat list entry (absolute path for -safe 0)
+  abs=$(cd "$(dirname "$f")" && pwd)/$(basename "$f")
+  printf "file '%s'\n" "$abs" >> "$concat_list"
+
+  # Probe duration
+  dur=$(ffprobe -v error -show_entries format=duration -of csv=p=0 "$f")
+  [ -z "$dur" ] && die "Could not read duration of: $f"
+  dur_ms=$(awk "BEGIN { printf \"%d\", $dur * 1000 }")
+
+  # Chapter title from filename (strip path and extension)
+  title=$(basename "$f")
+  title="${title%.*}"
+
+  end_ms=$((cumulative_ms + dur_ms))
+  cat >> "$metadata" <<EOF
+
+[CHAPTER]
+TIMEBASE=1/1000
+START=$cumulative_ms
+END=$end_ms
+title=$title
+EOF
+  cumulative_ms=$end_ms
+done
+
+# --- Concatenate ---
+
+ffmpeg -y -f concat -safe 0 -i "$concat_list" -i "$metadata" \
+  -map_metadata 1 -c copy -movflags +faststart "$output" 2>&1
+
+# --- Summary ---
+
+total_secs=$((cumulative_ms / 1000))
+mins=$((total_secs / 60))
+secs=$((total_secs % 60))
+
+echo ""
+echo "Done: $output"
+echo "  Chapters: ${#inputs[@]}"
+echo "  Duration: ${mins}m ${secs}s"