docs(changelog): clarify Unreleased section is finalized, point at v0.5.1 plan

Addresses Gemini-medium suggestion on PR #18: the prior phrasing
'(no unreleased changes — next minor will pick up Phase 14, tracked in
docs/roadmap.md)' was ambiguous about whether the v0.5.0 release was
finalized vs. still mid-flight. Reword to 'Empty — v0.5.0 is finalized.
The next minor (v0.5.1) covers Phase 14 ...' so the section state is
explicit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: cemililik

CHANGELOG.md

@@ -4,8 +4,9 @@ All notable changes to ForgeLM are documented here.
 
 ## [Unreleased]
 
-(no unreleased changes — next minor will pick up Phase 14, tracked in
-[docs/roadmap.md](docs/roadmap.md))
+_Empty — v0.5.0 is finalized. The next minor (`v0.5.1`) covers Phase 14
+(Multi-Stage Pipeline Chains); see [docs/roadmap.md](docs/roadmap.md)
+for the active phase plan._
 
 ---