perf(early-kickout): base seed blacklist on anchor last-final block

[stedfn]

Stacked on #16049 (thresholds). Follow-up to the early-kickout writer.

The record-block seeder computed the chunk-producer blacklist from the aggregator walked up to the unfinalized anchor. During a finality stall that rescans the growing not-yet-finalized suffix on every block, an O(stall-depth) per-block walk under the epoch-manager write lock.

This bases the blacklist on the anchor's last-final block instead (BlockInfo::last_final_block_hash / last_finalized_height).

• Deterministic: the last-final hash is copied verbatim from the block header, so it is identical across nodes for a given canonical anchor. Required because the seeded row is read on the strict consensus path (get_chunk_producer_info_anchored) with no recompute.
• Bounds the walk: the aggregator's sync point already sits at the last-final block, so the walk covers only blocks finalized since the previous record (~1 in steady state, 0 during a stall) and never the unfinalized suffix.
• Grace window now gates on the last-final block height too, matching the blacklist basis so the seeder is internally consistent.
• get_chunk_producer_blacklist accessor updated to the same basis.

Tests: model 2-block finality in the block helper, shift the grace-boundary anchors, and retighten the finality-stall walk guard from O(depth^2) to constant-per-block across two stall depths.

Stale/old-fork anchors whose last-final is an ancestor of the cached sync point still walk (rare, non-canonical, bounded by epoch length). A protocol-fixed budget cap for that case is left as a follow-up.

[codecov]

Codecov Report

❌ Patch coverage is 83.87097% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.21%. Comparing base (657a026) to head (bd0e342).

Files with missing lines	Patch %	Lines
chain/epoch-manager/src/adapter.rs	66.66%	1 Missing and 2 partials ⚠️
chain/epoch-manager/src/lib.rs	90.90%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@                       Coverage Diff                        @@
##           stedfn/early-kickout-thresholds   #16072   +/-   ##
================================================================
  Coverage                            73.20%   73.21%           
================================================================
  Files                                  856      856           
  Lines                               188485   188499   +14     
  Branches                            188485   188499   +14     
================================================================
+ Hits                                137979   138006   +27     
+ Misses                               46086    46068   -18     
- Partials                              4420     4425    +5     

Flag	Coverage Δ
pytests-nightly	1.22% <0.00%> (-0.01%)	⬇️
unittests	69.86% <47.05%> (+0.01%)	⬆️
unittests-nightly	69.86% <82.75%> (-0.01%)	⬇️
unittests-spice	65.46% <82.75%> (-0.02%)	⬇️

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