Roadmap: stabilize stdlib surface and generate API index

[zrr1999]

Problem / current limitation

The shipped stdlib is useful, but namespace shape, module stability, and user-facing API discovery are still not formalized enough to claim a stable standard library contract.

Evidence

• spore-evolution/seps/SEP-0009-standard-library.md still contains future-facing namespace/stability material.
• The release truth pass flagged stdlib stability and namespace alignment as incomplete.

Target behavior

• Define the supported stdlib namespace and which modules are shipped vs experimental.
• Generate an API index/reference from the checked stdlib.
• Add explicit stability caveats where APIs may still move.

Acceptance tests

• Docs list shipped modules and stability level.
• Generated API index stays in sync with the checked stdlib.
• CI fails if documented/generated stdlib inventory drifts from shipped modules.

Release priority

P1 / roadmap after first public release