parity(slurm): implement burst-buffer staging subsystem (BurstBuffer* reasons)

[yansun1996]

Summary

The BurstBufferResources and BurstBufferStageIn pending reasons exist as vocabulary (landed in #301) but have no producer, because Spur has no burst-buffer subsystem. Today --bb/burst-buffer is only script-wrapping in the node agent — crates/spurd/src/executor.rs::wrap_with_burst_buffer prepends stage-in/out shell commands to the job script. There is no resource pool, no staging state machine, and no scheduler awareness.

This is a self-contained subsystem spanning the scheduler, the node agent, and storage — its own epic, and a likely-deferred item relative to the rest of the Category-4 lifecycle work.

Gap / work

A real burst-buffer implementation (à la Slurm's burst_buffer plugin) needs:

• Resource pool: track burst-buffer capacity cluster-wide (config + accounting), allocate/free per job.
• Stage-in / stage-out state machine: a job requesting BB enters a staging phase before/after running; the controller models the staging lifecycle.
• Scheduler hold-until-staged: jobs wait in PENDING with Reason=BurstBuffer* while capacity is unavailable (BurstBufferResources) or data is staging in (BurstBufferStageIn), and are not dispatched until staging completes.
• Agent-side data movement: spurd performs the actual stage-in/out (beyond the current script-wrapping passthrough), reporting staging progress/completion to the controller.

Acceptance Criteria

• Burst-buffer capacity is modeled and consumed per job
• A job blocked on BB capacity shows Reason=BurstBufferResources; one staging in shows Reason=BurstBufferStageIn
• Jobs are not dispatched until stage-in completes; stage-out runs after the job
• Tests drive the real staging path

Notes

• Effort is large (resource pool + state machine + scheduler integration + agent data movement); realistically a multi-PR epic.
• Likely deferred until prioritized — flagged here so the two vocabulary-only reasons have a tracked home.

Related

• feat(spur): expand pending-reason vocabulary and surface reservation/license/QoS reasons #301 — landed the BurstBuffer* reason strings (vocabulary only).
• parity(slurm): wire emission logic for pending-reason vocabulary landed in #301 #307 — pending-reason emission umbrella (this is its Group 4).