Roadmap: implement structured concurrency runtime subset from SEP-0007

[zrr1999]

Problem / current limitation

The language design references spawn, await, parallel_scope, channels, and cancellation semantics, but the current runtime support remains a subset and should not be advertised as full SEP-0007 structured concurrency.

Evidence

• spore-evolution/seps/SEP-0007-structured-concurrency.md specifies richer runtime semantics than current validated behavior.
• The release truth pass identified cancellation, channel/select behavior, and deterministic handler/runtime semantics as roadmap-only.

Target behavior

• Implement and validate a minimal structured concurrency runtime with explicit semantics for task lifetime, cancellation propagation, and deterministic parallel_scope completion.
• Stage channel/select support behind documented scope boundaries if they are not part of the first slice.

Acceptance tests

• Runtime tests cover spawn/await lifecycle, nested scope cancellation, and deterministic cleanup.
• Docs distinguish implemented subset vs deferred channel/select features.
• Cost/effect annotations for concurrency constructs remain truthful.

Release priority

P1 / roadmap after first public release