fix: set base_form on form-locked evolutions

[MiquelRForgeFlow]

A few species only evolve from their default form, but their evolution rows in pokemon_evolution.csv left base_form empty (which reads as "any form"). This wrongly implies their permanent non-evolving alternate forms can also evolve:

• Pikachu's cap / cosplay / partner / starter forms
• Partner Eevee
• Eternal Flower Floette

These forms are ordinary (not is_battle_only) and have no evolution row of their own, so nothing else disambiguates them. This PR sets base_form so only the default form evolves.

It also fixes Pumpkaboo → Gourgeist, which is size-preserving: the single trade evolution is split into four size-locked rows, each with base_form/evolved_form set so every Pumpkaboo size evolves into the Gourgeist of the same size (average/small/large/super).

Changes:

• Pikachu → Raichu: base_form = pikachu on both the Kanto and Alola Raichu rows.
• Eevee → eeveelutions: base_form = eevee on all Eeveelution rows.
• Floette → Florges: base_form = floette.
• Pumpkaboo → Gourgeist: four size-locked rows (average/small/large/super) with matching base_form/evolved_form.

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