[Refactor]: Extract decay JSON parsing to remove “ignore/keep default” comments

[AashishH15]

Problem

UI decay parsing swallows parse failures with “ignore/keep default” comments, and logic is duplicated.

Evidence

• DecayDashboard swallows parse failures with // ignore at DecayDashboard.tsx (line 18). NodeEditor has the same pattern with // keep default at NodeEditor.tsx (line 300).

Scope & constraints

No response

Proposed approach

Create shared helpers:

• parseDecayStateOrDefault, getDecayRate, getDecayScore, setDecayRate

Acceptance criteria

• No duplicated JSON parsing across components.
• Failure behavior consistent and explicit.

Test plan

• UI smoke test for dashboard/editor decay controls.