fix(ecfinality): account for null rounds in chain walk

[rvagg]

Include null round epochs as 0-block entries in the chain data fed to the FRC-0089 calculator, rather than silently skipping them. This aligns the implementation with the theoretical model where null rounds are epochs where the adversary gains ground while the honest chain doesn't grow. Also fixes a depth-to-height conversion problem which previously could return a shallower tipset than intended when null rounds were present between head and the finalized epoch.

Increase BisectHigh from 200 to 450 to accommodate degraded chains where null round inclusion pushes the threshold deeper (notably current calibnet which is near 200).

Thanks to @hanabi1224 for questioning the implementation on this point while doing ChainSafe/forest#6811, and @guy-goren for confirming that null's are statistically important to include.

Current mainnet shows no difference in the calculation because nulls are so rare, but the difference is ~5 epochs on calibnet where we're averaging 2.2 blocks per tipset and have a lot more null rounds.

[Copilot]

Pull request overview

Updates the EC finality chain-walk logic to include null rounds as explicit 0-block epochs so the FRC-0089 calculator operates over a true epoch timeline (and depth maps cleanly to height), and expands the bisect search range to handle more degraded chains.

Changes:

• Include null-round epochs as 0 entries when walking back the chain (both in lotus-shed finality-calculator and in the in-node ECFinalityCache).
• Increase ecfinality.BisectHigh from 200 to 450 and update related documentation/comments.
• Update changelog and adjust test comments accordingly.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
cmd/lotus-shed/finality.go	Inserts 0-block epochs for null rounds during API chain walk; trims to fixed window and preserves epoch-to-depth correctness for output.
chain/ecfinality/cache.go	Updates cache chain walk to include null rounds as 0 entries so returned depth aligns with epoch height differences.
chain/ecfinality/calculator.go	Expands bisect search upper bound from 200 to 450.
chain/ecfinality/calculator_test.go	Updates test comment to reflect new BisectHigh.
CHANGELOG.md	Adds an UNRELEASED entry documenting the null-round handling fix and associated depth/height correction.

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