refactor(spice): tidy chunk-certification endorsement bookkeeping (1/6)

[darioush]

Two behavior-preserving refactors of the SPICE chunk-certification code.

Tally helper:

• Extract the per-result endorsement-threshold tally shared by the live and on-chain certification paths into a single reaches_endorsement_threshold helper on SpiceCoreReader, replacing the duplicated inline assignment-lookup-and-tally in core_writer_actor.
• Add is_endorsed on ChunkValidatorAssignments so callers read the verdict directly.

Block-validation endorsement state:

• Collect the waiting_on_endorsements / known_endorsements maps in validate_core_statements_in_block into an AncestryEndorsements struct that owns the "endorsement state as of the previous block" bookkeeping.
• Extract push_stored_endorsements and verify_endorsement_signature from core_statements_for_next_block / validate_core_statements_in_block.
• Add an as-yet-unwritten present_fallback_endorsements field to SpiceUncertifiedChunkInfo, read through all_present_endorsements(), so later work can accumulate endorsements without re-touching the validation loop.

Note: Full change (non-stacked PRs) is available here: #15935

[codecov]

Codecov Report

❌ Patch coverage is 0% with 137 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.68%. Comparing base (cf425fe) to head (5c85509).

Files with missing lines	Patch %	Lines
chain/chain/src/spice/core.rs	0.00%	66 Missing ⚠️
chain/chain/src/spice/ancestry_endorsements.rs	0.00%	34 Missing ⚠️
chain/chain/src/spice/core_writer_actor.rs	0.00%	22 Missing ⚠️
...s/src/stateless_validation/validator_assignment.rs	0.00%	10 Missing ⚠️
core/primitives/src/types.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15936      +/-   ##
==========================================
- Coverage   69.70%   69.68%   -0.03%     
==========================================
  Files         946      947       +1     
  Lines      204272   204311      +39     
  Branches   204272   204311      +39     
==========================================
- Hits       142395   142381      -14     
- Misses      57124    57183      +59     
+ Partials     4753     4747       -6     

Flag	Coverage Δ
pytests-nightly	1.10% <0.00%> (-0.01%)	⬇️
unittests	69.43% <0.00%> (-0.03%)	⬇️
unittests-nightly	69.38% <0.00%> (-0.02%)	⬇️

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[Copilot]

Pull request overview

This PR refactors SPICE chunk-certification endorsement bookkeeping by extracting duplicated endorsement-threshold logic and encapsulating “previous block endorsement state” into a dedicated helper struct, with a preparatory field added to track fallback endorsements.

Changes:

• Adds SpiceCoreReader::reaches_endorsement_threshold and ChunkValidatorAssignments::is_endorsed to reuse the endorsement-threshold tally logic.
• Introduces AncestryEndorsements to centralize “pending/known endorsements from previous uncertified chunks” handling during block core-statement validation.
• Extends SpiceUncertifiedChunkInfo with present_fallback_endorsements and provides all_present_endorsements() for unified iteration.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
core/primitives/src/types.rs	Adds fallback-endorsement storage + iterator helper on SpiceUncertifiedChunkInfo.
core/primitives/src/stateless_validation/validator_assignment.rs	Adds is_endorsed stake-verdict helper to avoid materializing full endorsement state.
chain/chain/src/spice/tests/core.rs	Updates test helper struct construction for new SpiceUncertifiedChunkInfo field.
chain/chain/src/spice/mod.rs	Registers new ancestry_endorsements module.
chain/chain/src/spice/core.rs	Uses AncestryEndorsements; extracts signature verification + stored-endorsement push; adds threshold helper.
chain/chain/src/spice/core_writer_actor.rs	Replaces duplicated store-endorsement tally logic with reaches_endorsement_threshold.
chain/chain/src/spice/ancestry_endorsements.rs	New struct encapsulating previous-block endorsement bookkeeping for validation.

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