feat: add configurable player attributes for Psi capacity and regeneration

[lightsing]

Summary

This PR adds two player attributes—Total Psi and Psi Regeneration—so other mods can interact with Psi’s resource economy via standard Minecraft/NeoForge attribute mechanics (modifiers from equipment, effects, professions, etc.). With Psi alone installed, behavior is unchanged. Defaults: psi:total_psi = 5000, psi:regen = 25.

Motivation

Previously, the total pool and regeneration rate were hardcoded in PlayerDataHandler, which made inter-mod extensions require mixins or other invasive hooks. Exposing them as attributes lets integrators use the usual attribute modifier pipeline (effects, gear, professions, advancements, data-driven systems).

Non-goals

• No changes to spell logic, CAD behavior, or energy math beyond sourcing totals/regen from attributes.
• No new config or UI.

Backward compatibility

• No gameplay change by default: defaults match previous constants (Total Psi = 5000, Regen = 25).
• Save-compatible: attributes apply default values automatically; no data migration required.

Testing

• Client and server with only Psi: pool and regeneration unchanged.
• With attribute modifiers applied: capacity/regen update as expected.

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Changelog (for modders)

Added player attributes psi:total_psi and psi:regen. PlayerDataHandler now reads these to determine Psi capacity and regeneration. Defaults preserve prior behavior. Integrate via standard attribute modifiers.

[Kamefrede]

LGTM, thank you