Roadmap: expand native backend beyond current project/object subset

[zrr1999]

Problem / current limitation

Native build support now works for standalone files and project-aware object generation, but that is still narrower than the long-term compiler/backend story described across the roadmap and SEP materials.

Evidence

• The recent native build work delivers .o output and project target awareness.
• spore-evolution/ROADMAP.md and SEP-0006 still describe broader backend, diagnostics, and toolchain goals.

Target behavior

• Define the next supported native milestones after object generation: linking strategy, platform coverage, diagnostics parity, and integration with project targets.
• Keep unsupported backend targets and self-hosting milestones clearly roadmap-only.

Acceptance tests

• Compiler tests cover the next chosen native slice (for example linked executable flow or broader target coverage).
• Docs distinguish experimental backend capabilities from supported release behavior.
• CLI help and site docs stay aligned with actual backend support.

Release priority

P2 / roadmap after first public release