CI: split Build / Test / Pack(+Release) into discrete stages

[ChrisonSimtian]

Today CI runs dotnet fallout Test Pack as one step. Split into discrete Build → Test → Pack(+Release) stages/targets so:

• experimental can do build + unit-test only cheaply (then push GH Packages),
• failures are attributable (compile vs test vs pack),
• pack/publish is its own gated stage.

Structural — coordinate with the 3-tier realignment.

[ChrisonSimtian]

Partial: test-before-publish landed on all lanes (#331). Discrete per-CI-step Build/Test/Pack intentionally NOT split — NUKE re-runs the dependency graph per dotnet fallout invocation (double-compile), so a single Test Pack IS the staged build. Leaving open only if a true job-per-target split (artifact handoff) is still wanted; otherwise closeable.