0.5.5

Document the 0.5.4 API additions

Describe what the stable API gained in 0.5.4, in game-version terms rather
than internal contract levels. A modder already knows which game version
their players are on; the contract level was a second version axis that only
ever restated the same fact, and the reference now marks each addition with
the version that introduced it.

Damage and shields: deal_damage now runs the game's full attack pipeline, so
it applies resistances and crit, records damage statistics, awards kill and
assist credit, procs items and buffs, and shows the damage number. This is
the one change that reaches already-built modules, so the page says so
explicitly and points at deal_damage_raw for anyone who wanted the old
unmitigated subtraction. Shields get their own grant/clear calls, because a
shield is an absorb layer rather than a buff and putting shield numbers in a
BuffV1 quietly did nothing.

Items: document the callback set in full rather than the previous one-line
summary, including on_base_attack, on_assist, on_dead, on_cc, the upgrade
carry-over pair, the attack type and crit flag now carried by on_attack and
on_damaged, and the ally skill hits that on_skill_hit previously never
reported. Also clarify a long-standing confusion: the entity argument is the
owner's own champion, and the killed entity is now a separate argument.

Item builds: add StableItemBuildHook, which is how a mod gets its own items
actually built by the AI. The old category selection could only ever pick
base-game items.

Champions: add lane_prior, so a new champion is not read as equally playable
in all five lanes until it accumulates match history.

Compatibility: state plainly that building against the newest SDK is always
safe in both directions, and document requires = N as the rare opt-in for a
mod that would be inert on an older game and wants an update prompt instead.

Also refresh the classic reference, whose passive and item signatures moved
with the same change, and note that the new surfaces have no classic
equivalent.