op-batcher: throttle config overhaul (pending-DA defaults, always-limit, step alignment)

[sashass1315]

• Deprecate flag throttle.threshold-multiplier; keep backward-compat if --throttle.unsafe-da-bytes-upper-threshold is not set.
• Add throttle.tx-size-always-limit to cap tx size even at zero intensity.
• Add throttle.step-threshold-alignment (start|middle|end) to position the step threshold within the [lower, upper] range.
• Compute throttling intensity from pending DA bytes; still record unsafe DA bytes for visibility; update ratio metric to use pending bytes.
• Update defaults: lower=2,000,000 bytes (~1–2 channels), upper=lower20 (within 10–50x guidance).
• Controller: apply always-on tx limit at intensity 0; respect step alignment when selecting threshold.
• Docs: refresh op-batcher/throttling.md and op-batcher/readme.md with new flags, defaults, and guidance.
• Minor tests/docs comments adjusted for consistency.

Fix #16947