handlers: mission energy cost, level-up, and battle drops

[Seltraeh]

handlers: mission energy cost, level-up, and battle drops

Branch: split/13-mission-flow
Base: dev
Merge position: 13 of 13

Part of the PR #28 split. Each PR branches from dev and contains only its
own changes, so this diff is exactly one subsystem. The set is designed to be
merged in numeric order; merging all 13 reproduces PR #28 byte for byte
(verified against tree 79a4e065).

Later PRs in the series touch Handlers.hpp, GmeControllerHandlers.cpp and
UserInfo.cpp too, so once earlier ones land this branch may need a rebase.
Those conflicts are always additions on both sides — keep both. Maintainer
edits are enabled, so feel free to push the rebase directly to this branch.

Final PR. Moves MissionStart / MissionEnd onto real data.

What's included

MissionStart takes the energy cost from mission_mst keyed on the requested mission id instead of a fixed value. It also reads the mission id from its nested position in the request rather than the top level, where it was never present — the previous fallback made every mission launch as mission 10.

MissionEnd credits per-mission zel and karma from the battle result, applies level-ups by per-level chunk subtraction, and refills energy to the new level's cap on ding. Dropped units go through addUserUnit and dropped items through addUserItem, driving both the obtained-unit animation and the first-encounter encyclopedia unlock.

Known incomplete

Battle wave content is still the captured mission-10 enemy set. The wave headers are keyed off the requested mission so the client renders and plays correctly, but loading per-mission waves from mission_mst remains outstanding and is the obvious next task.

Verification

Start a mission: correct energy deducted. Complete it: rewards credited, level-up animation plays once without cascading, drops appear.