feat(#2485): remove vestigial Noop + StartupSweep residuals

Since the boot sweep (#2450) and meta-update cascade (#2476) became
single DAGs that grow subgraphs in-place, nothing constructs the old
fan-out anchors/parents anymore:

- NodeKind::Noop (the old boot_root grouping anchor) — no constructors.
- Template::StartupSweep / Source::StartupSweep (the old fan-out parent
  template + cascade-child source) — replaced by Template::Boot and
  Source::AutoUpdate/MetaUpdate respectively.

Drops the three variants + their as_str arms + the Noop executor arm, and
refreshes the stale fan-out/anchor doc comments (Boot/MetaUpdate/Source
docs, coordinator.md, dashboard.md). Frontend: the queue-kind glyph moves
from the dead startup_sweep to boot (which had none), and the dead
rqe-source-startup_sweep style is dropped.

No behaviour change — pure dead-variant removal.

frontend/packages/dashboard/src/system-sections.css

@@ -128,7 +128,6 @@
 .rqe-source-manual { color: var(--cyan); border-color: var(--cyan); }
 .rqe-source-meta_update { color: var(--purple); border-color: var(--purple); }
 .rqe-source-auto_update { color: var(--muted); }
-.rqe-source-startup_sweep { color: var(--muted); }
 .rqe-source-crash_recover { color: var(--amber); border-color: var(--amber); }
 .rqe-source-approval { color: var(--green); border-color: var(--green); }
 .rqe-when { color: var(--muted); font-size: 0.85em; }