v2.1.0: cobweb phasing fix, fall damage immunity, Serpent Aegis restrictions

Arachne

• Walking through cobwebs no longer slows you down. The power meant to do this had silently done
• nothing since the mod's very first commit, confirmed by decompiling Apoli directly, now fixed
• with the real, functional power.
• Sure Footed is now unconditional. She never takes fall damage at all, matching how the fall
• damage reduction power actually behaved in practice (it never really applied its intended
• partial reduction above 10 blocks).
• A normal punch (webbing a target in a temporary cobweb without sneaking) now applies Immobilized
• for 2 seconds, down from 3.
• An Immobilized target can no longer hit you if you are standing behind it, or from more than
• point blank range. Previously it could still land hits regardless of facing or distance.

Medusa

• Removed Stone Gaze, the passive, always on petrify pulse. Stone Gaze Burst, the secondary key
• ability, is unchanged.
• The Serpent Aegis is now genuinely Medusa only. Before, any origin able to use a shield at all
• could raise and block with it, despite the tooltip already claiming otherwise.
• The Serpent Aegis now actually slows its wielder while equipped, matching Slowness I. This
• needed a real attribute modifier rather than the status effect itself, since Medusa's own Stone
• Cursed immunity blocked the effect version entirely.

Mermaid

• Removed "every sea creature is friendly." Sea creatures behave like vanilla again.