Add BW2 in-game trades

[matthewchoy88]

Summary

Adds the seven specific NPC in-game trades from Pokémon Black 2 and White 2, sourced from Serebii.

Location	You Give	You Receive	Level	Versions
Route 4	Cottonee	Petilil	20	Black 2 only
Route 4	Petilil	Cottonee	20	White 2 only
Route 7	Emolga	Gigalith	35	Both
Humilau City	Mantine	Tangrowth	45	Both
Route 15	Ditto	Rotom	60	Both
Accumula Town	Excadrill	Ambipom	40	Both
Accumula Town	Hippowdon	Alakazam	40	Both

Changes

Modeled on the existing GSC (#1473) and RSE (#1478) trade PRs:

• encounter_slots.csv: new slot 1482 for the BW2 npc-trade method (vg 14, method 36).
• location_areas.csv / location_area_prose.csv: new location area 1363 for Accumula Town — no entry existed.
• encounter_condition_values.csv / encounter_condition_value_prose.csv: seven new trade-<species> condition values (trade-cottonee, trade-petilil, trade-emolga, trade-mantine, trade-ditto, trade-excadrill, trade-hippowdon) with English prose.
• encounters.csv: 12 new encounter rows (6 per version) with min_level = received level and max_level = 100, following the convention from prior trade PRs.
• encounter_condition_value_map.csv: 12 new rows linking each encounter to its "give" condition.

Out of scope

The Yancy/Curtis Funfest Mission trades in Nimbasa City are not included. They accept any Pokémon and branch on player gender, which would need new encounter-condition infrastructure (no trade-any or gender condition exists today). Happy to do these in a follow-up if there's interest.

AI Declaration

Change was made with the help of an AI coding agent.

Contributor check list

• I have written a description of the contribution and explained its motivation.
• I have written tests for my code changes (if applicable).
• I have read and understood the AI Assisted Contribution guidelines.
• I will own this change in production, and I am prepared to fix any bugs caused by my code change.