Iteration 11 — fibers (green threads on the shard scheduler)

Format: product/story-iteration-template. Part of Story — one language, one runtime, one database, one binary.

Goals

Concurrency inside a shard the doctrine way: cooperative fibers scheduled by the VM — a fiber is the interpreter state wovm already isolates (register windows + frame stack + pc), made per-fiber. One kernel task per core stays the rule; thousands of fibers run above it where the kernel tops out at hundreds of threads (CFS O(log N), ~8 MiB + ~20 µs per thread vs an arena-allocated fiber context).
Preemption by reduction budget, the Erlang shape: the dispatch loop counts down and parks the fiber at zero — no signals, no safepoints, no stack copying, deterministic replay.
Blocking builtins park instead of block on server shards: the same net/time/fs calls that block the thread in program mode hand their fd to the shard's epoll/io_uring loop and resume on completion. One API, same source text, no async/await, no function coloring — ever.
Cross-fiber sends follow the iteration 8 rule — ownership moves — on the cheaper same-heap path.

What to achieve?
- Given a shard running thousands of spawned fibers with one hot compute loop among them,
- when the reduction budget expires,
- then the hot fiber parks, every other fiber makes progress (starvation-free corpus fixture), and output is identical across runs (deterministic scheduling).
What to achieve?
- Given a fiber blocked in a stdlib builtin on a server shard,
- when the completion arrives on the shard's event loop,
- then the fiber resumes with the result while the shard served other fibers in the interim — one OS thread, verified by TID.
What to achieve?
- Given a parked fiber at shard shutdown,
- when the shard unwinds it,
- then every drop map runs (ASan zero leaks) — parked fibers die as cleanly as trapped ones. (Iteration 12's blue-green drain reuses exactly this unwind path.)
What to achieve?
- Given @gc objects referenced only from a parked fiber's frames,
- when the per-shard cycle collector scans,
- then fiber stacks are roots — nothing live is collected, nothing dead survives.

Fiber migration across shards (ownership doctrine forbids it; cross-shard is a message send, iteration 8).
Priorities, timers, structured-concurrency policy — recipe-box capabilities for a later .wo library, not runtime policy.
Program mode: stays single-fiber in v1 (blocking legal, log-watcher needs nothing more).
async/await keyword — permanently rejected surface, not deferred.

Research note: docs/plan/exploration/fibers/00-fibers.md — kernel's-eye evidence (task_struct costs, CFS collapse at high task counts) and the precedent survey (BEAM reductions adopted; Go stack copying and Tokio coloring rejected; Loom's park-under-blocking-API matches the stdlib posture).
Vision origin: blue-green vision §3; iteration 8's scheduler is the substrate this extends.
Open questions to settle in the spec: spawn surface (handle vs actor address), budget size and check granularity, parked-fiber drop semantics, run-queue fairness (FIFO v1).

No implementation plan exists yet — this iteration starts with the brainstorming → spec → writing-plans chain (the superpowers path every prior iteration followed), then executes that plan. The research note above is the brainstorm's entry material.