Clear invalid spell selections when converting Hellfire saves to Diablo

[morfidon]

Closes #3466

Summary

When a Hellfire save is converted to Diablo while a Hellfire-only spell is selected, that selection can survive in an invalid state.

In DevilutionX this can leave the player with a selected spell that has no icon but may still remain usable. In vanilla Diablo, loading such a save can crash.

This change clears invalid spell selections during load/conversion and keeps the player's spell state consistent afterwards.

What changed

• call ValidatePlayer() before recalculating inventory-derived spell state during load
• use CalcPlrInv() during loading so spell sources from inventory, scrolls, and charges are rebuilt before validating selections
• update spell selection validation so SpellType::Spell is no longer treated as automatically valid
• sanitize invalid selected spells and invalid hotkeys after loading
• normalize derived runtime spell state after sanitization so stale queued/executed spell data does not survive
• make LoadHotkeys() explicitly handle both legacy hotkey data and missing or malformed hotkey blobs safely

Why this approach

This issue is triggered by Hellfire -> Diablo conversion, but the real problem is broader than a single Hellfire spell id surviving the rewrite.

After conversion, the selected spell and hotkeys may no longer be valid for the rebuilt player state. Because of that, this patch does not special-case only Hellfire spell ids. Instead, it validates spell selections against the player's actual available spell sources after loading:

• learned spells
• skills
• scrolls
• charges

This follows the intent discussed in #3466:

• validate the player before recalculating spell-related state
• rebuild spell availability from inventory
• clear invalid readied spells
• do the same for LoadHotkeys()

The extra runtime-state normalization is intentional. Clearing only _pRSpell / _pRSplType would still leave stale queuedSpell, executedSpell, or spellFrom state behind, which could keep the player in a partially invalid state after sanitization.

Updated load/conversion flow

Now, during Hellfire -> Diablo conversion:

1. the player is loaded
2. ValidatePlayer() removes invalid Hellfire-only learned spell state
3. CalcPlrInv() rebuilds spell availability from inventory, including scrolls and charges
4. hotkeys and the selected spell are loaded
5. any selection that is no longer valid for the rebuilt player state is cleared
6. runtime spell state is synchronized from the sanitized selection

This ensures that invalid Hellfire spell selections do not survive conversion as selected, queued, or executed spell state.

Tests

Added tests to cover:

• spell selection validation by source
• rejecting invalid spell/type combinations
• clearing invalid spell selections when reading from save
• sanitizing and normalizing state when the hotkeys file is missing
• sanitizing invalid selections from legacy hotkey data
• preserving valid legacy scroll selections
• persisting sanitized selections after Hellfire -> Diablo rewrite