[Refactor]: Convert decay test section comments into nested test modules

[AashishH15]

Problem

Test grouping is currently done with banner comments; harder to navigate.

Evidence

decay.rs (line 98), line 170 (line 170), and line 232 (line 232) use banner comments to group tests.

Scope & constraints

No response

Proposed approach

Use nested test modules (e.g., mod score_tests, mod active_rollover_tests, mod frozen_rollover_tests).

Acceptance criteria

• No semantic changes to tests.
• Navigation improved.

Test plan

• cargo test.