fix(control): preserve explicit zero battery limits

[frahlg]

Summary

• Preserve omitted battery limits separately from an explicit 0 W limit.
• Carry that distinction from the config adapter through allocation and every control clamp.
• Treat a one-sided zero as a closed direction while keeping legacy positive PowerLimits callers unchanged.
• Keep the current exact 0/0 config-error fallback instead of disabling the whole battery.

Root cause

batteries.<id>.max_charge_w: 0 and max_discharge_w: 0 used pointer fields, but driverLimitsFrom copied only values above zero. Control then read zero as “unset” and fell back to MaxCommandW (5 kW). The planner could forbid a direction while runtime still commanded it.

The shared cap rule now applies to allocation, SoC clamps, post-slew clamps, charge mode, PV headroom and the fuse saver. If discharge is explicitly disabled, the fuse saver will not use that battery; another device or safety layer must handle any remaining import overage.

Tests

• go test -race -count=1 ./internal/control ./cmd/ftw
• pre-commit make verify
• pre-push make verify-all, including optimizer integration and Linux/Windows cross-builds

Stack and scope

This PR is stacked on the exact reviewed head of #769 because both PRs touch main.go and dispatch.go. Retarget it to master after #769 lands.

This does not change config validation or per-storage optimizer-to-dispatch assignment.

Refs #885
Related: #816