Add db-migrations-expand-contract: zero-downtime schema migrations proven with a real 4-deploy rolling run

Companion code for Zero-Downtime Database Migrations: Expand-Contract in Practice
with Spring Boot: a full expand/migrate-writes/migrate-reads/contract sequence run
as an actual rolling deploy across two live replicas, with a load generator sending
continuous HTTP traffic through all four deploys (99.98% success, every residual
error traced to a root cause rather than left unexplained). Findings include a real
NOT NULL constraint trap in the expand migration, a backfill-window bug in the read
switch, H2's AUTO_SERVER=TRUE single-point-of-failure behavior under a rolling
restart, the drain-before-SIGTERM fix needed to close a health-check gap during
graceful shutdown, and H2 silently discarding a concurrently committed INSERT during
an ALTER TABLE ADD/DROP COLUMN rebuild - confirmed, by primary source, to be an
H2-specific behavior rather than a property of the technique itself.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_019Fb7vW8vLyLKngBc4R3huA

db-migrations-expand-contract/scripts/start-instance.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+# Starts one replica on a given port and stage, and waits for it to report healthy.
+# Usage: start-instance.sh <port> <stage>
+set -euo pipefail
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+source "$DIR/env.sh"
+
+PORT="$1"
+STAGE="$2"
+
+# -XX:TieredStopAtLevel=1 and a small fixed heap cut this JVM's own startup CPU
+# burst dramatically (C2 compilation and heap sizing are the two biggest costs of a
+# cold Spring Boot start). That matters here specifically because two replicas share
+# a 2-core sandbox: a slow-starting replica can starve its own sibling's health
+# checks long enough to look like an outage that never actually happened - see
+# docs/11-the-load-generator.md.
+nohup java -XX:TieredStopAtLevel=1 -XX:+UseSerialGC -Xms128m -Xmx256m \
+    -jar "$JAR" --server.port="$PORT" --app.stage="$STAGE" \
+    > "$EC_LOG_DIR/instance-$PORT.log" 2>&1 < /dev/null &
+echo $! > "$EC_PID_DIR/$PORT.pid"
+echo "started stage $STAGE on port $PORT (pid $(cat "$EC_PID_DIR/$PORT.pid"))"
+wait_healthy "$PORT"
+echo "port $PORT healthy"