feat(spice): epoch transition with uncertified chunks last_proposal

[darioush]

• In SPICE, validator proposals come from execution results (not chunk headers). Block production and validation now source proposals from SpiceCoreStatement::ChunkExecutionResult entries instead of chunk headers.
• At epoch boundaries, staking proposals from uncertified chunks are included in last_proposals to prevent incorrect stake return. Without this, a validator who re-stakes late in an epoch could have their locked stake incorrectly returned.
• Refactored block proposal validation into a validate_block_proposals helper that handles both SPICE and non-SPICE blocks.

[codecov]

Codecov Report

❌ Patch coverage is 20.00000% with 52 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.80%. Comparing base (3886ec4) to head (d61d167).

Files with missing lines	Patch %	Lines
chain/chain/src/spice_core.rs	0.00%	32 Missing ⚠️
chain/chain/src/chain.rs	52.17%	11 Missing ⚠️
chain/client/src/chunk_executor_actor.rs	0.00%	8 Missing ⚠️
core/primitives/src/block.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #14980       +/-   ##
===========================================
+ Coverage    1.30%   68.80%   +67.50%     
===========================================
  Files         725      919      +194     
  Lines      140594   201207    +60613     
  Branches   140594   201207    +60613     
===========================================
+ Hits         1835   138447   +136612     
+ Misses     138706    56790    -81916     
- Partials       53     5970     +5917     

Flag	Coverage Δ
pytests-nightly	1.30% <0.00%> (-0.01%)	⬇️
unittests	68.43% <20.00%> (?)
unittests-nightly	68.36% <20.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.