fix(control): keep slew behind meter clamp

[frahlg]

What changed

• Keep the final battery fleet command inside the meter clamp intent.
• Preserve normal per-device slew where it is already safe.
• Project mixed fleets to the aggregate safe bound without depending on driver order.
• Add charge, discharge, mixed-fleet and ordinary-slew regression tests.

Why

The old order could reopen an upstream safety clamp. In the reported charge case, the meter clamp requested about +935 W but slew plus the command cap produced +5000 W. This change keeps the final fleet total at the safe meter intent.

Stack

This draft targets #922 and must merge after #926 and #922. It does not change either parent branch.

Closes #816.

Verification

• focused control regressions: 100 runs
• focused race regressions: 25 runs
• go test -race ./internal/control ./cmd/ftw
• full make verify
• git diff --check