fix(mock): guard the invoice amount against overflow, and complete the 0.3.2 changelog

[tcheeric]

Summary

Two small follow-ups from the post-merge review of 0.3.2 (#64, #65).

1. Overflow. MockLnServer.bolt11Amount computes amountSat * 10 on a value parsed from the request body without bounds. Above Long.MAX_VALUE / 10 it wraps silently, producing a negative or nonsense prefix. Refuse amountSat > 2_100_000_000_000_000 (21M BTC in sats) with a 400, or use Math.multiplyExact and refuse on overflow. The encoding itself is correct (1 sat = 10n, amountless for <= 0), and the bech32 checksum covers the new HRP.
2. Changelog. The 0.3.2 entry does not mention the lombok 1.18.42 / jackson-databind 2.20.0 bumps (chore(deps): lombok 1.18.42, jackson-databind 2.20.0 #65), or that phoenixd-mock now inherits the managed jackson version instead of pinning 2.17.0. The existing 0.3.1 heading also uses an em dash (## 0.3.1 — 2026-09-23), which the house style avoids.