Add Flux scheduler (parser + emitter)

[stackedsax]

Add support for the Flux scheduler (fluxrm/flux-sched), one of the two remaining planned HPC schedulers.

Scope

• internal/parser/flux — Flux jobspec JSON → SPLAT
• internal/emitter/flux — SPLAT → Flux jobspec JSON
• Register in both internal/{parser,emitter}/all/all.go; drop from the "Planned" list in cmd/bammm/main.go and mark ✅ in the README table

Known hard bits (see SPEC.md / CLAUDE.md)

• Flux symbolic named dependencies — no portable equivalent; preserve in extensions.flux, drop-with-warning cross-scheduler
• Flux NUMA/socket resource graph — flatten to total CPU count for non-Flux targets
• Priority via --urgency; add a PriorityScale in internal/splat/priority.go

Process: follow docs/adding-a-scheduler.md. Add a corpus config entry and check coverage against real specs; add unit + round-trip + golden tests.