Add CHANGELOG.md with v0.3.0 release notes

[Copilot]

No changelog existed in the repo; release history was only in GitHub Release bodies. Creates CHANGELOG.md documenting all releases (v0.0.1–v0.3.0) with v0.3.0 as the primary focus.

v0.3.0 changes since v0.2.0

• New: SciMLBase.remake for ConstantRateBranchingProblem — accepts lifetime/nchild for direct field replacement; unknown kwargs forwarded to inner problem's remake (works for both SDEProblem and JumpProblem)
• Deprecated: remake_initial_condition — superseded by remake
• Dependency: JumpProcesses promoted from test-only to full package dependency

remake(bp, nchild=3)                              # modify branching fields
remake(bp, u0=1.0)                                # forward to inner problem
remake(bp, lifetime=Exponential(0.5), tspan=(0.0, 4.0))  # combined

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.