block duration always late by ~10ms

[mmcgee-jump]

We target 350ms in Firedancer, but somehow even the local view always shows 360ms

[jherrera-jump]

The extra 10ms comes from

• ~7ms tower fork choice
• ~1ms prepare_leader_bank
• ~4ms pack slot_done timing + poh tail-hashes

The plan is to fix the ~4ms by dynamically lowering the microblock upper bound throughout the slot. The remaining delay we will leave as is and optimize it down as a separate issue.