Add unit tests for decay curve behavior #26

Summary

pkg/decay/curves.go currently has no direct unit tests, even though it contains core salience decay math used by schedulers.

Why this is a good first issue

• Small, self-contained file.
• Pure functions (no DB/network dependencies).
• Straightforward table-driven tests.

Proposed work

Add pkg/decay/curves_test.go with tests for:

• Exponential decay with positive half-life.
• Linear decay with positive half-life.
• Min-salience floor behavior.
• halfLife <= 0 fallback behavior.
• GetDecayFunc dispatch for known and unknown curves.

Acceptance criteria

• New tests pass with go test ./pkg/decay.
• Test names clearly describe expected behavior.
• Edge cases (halfLife <= 0, flooring) are covered.