Support pricing schedule changes via contract upgrade with announcement window

[timfong888]

Context

Pricing changes (data set floor, per-piece fees, sybil fee, axlUSDC accommodations) currently require a contract change. Rather than amend pricing fields directly each time, the contract should support a pricing schedule that announces upcoming changes via the upgrade mechanism.

This came out of the Pricing Proposal review (Apr 28–29, 2026) where Tim, Rod, Luca, and BigLep aligned that changing pricing means new engineering — but if we build a schedule mechanism once, future pricing changes become an upgrade announcement rather than a fresh contract change each time.

Requirements

• Contract holds a forward-dated schedule of price changes
• Upgrade mechanism announces changes coming (gives SDK + customers lead time)
• Schedule surfaced via Costs API so SDK consumers can read upcoming pricing
• Coordinate with SDK-side Costs API rework (separate ticket to follow)

Related

• XFN-298 (sybil fee → auction-based burn)
• FIP-115 (could flip premium/base-fee ratio — affects pricing assumptions)
• Luca to document rationale for the pricing changes themselves

Owner

Rod (committed to filing during the pricing proposal review thread)

[timfong888]

This is linked to FilOzone/synapse-sdk#763 re: SDK support

[rvagg]

Adding #469 as the parent here, leaving this open as something to make sure gets done but assuming it'll be done as part of #469 rather than as a discrete thing so estimate: 0.