feat(spice): add certified light-client header fields (schema)

[darioush]

Adds the two spice light-client fields to the block header and threads them through, with placeholder (default) production — the real values come from the certified accumulator in a later PR of this stack (marked TODO(spice) in client.rs).

• BlockHeaderInnerLiteV2 (= classic inner-lite + certified_block_merkle_root + last_certified_block); BlockHeaderV7 switches to it; compute_hash_and_sign generalized over the inner-lite type.
• Replaces the unified inner_lite() accessor with certified_block_merkle_root() / last_certified_block() Option accessors.
• BlockHeaderInnerLiteView gains the two fields as #[borsh(skip)] + serde (keeps the persisted EpochLightClientBlocks layout intact); LightClientBlockLiteView::hash() is version-aware.
• Threads the fields through Block::new, SpiceNewBlockProductionInfo, BlockHeaderView, TestBlockBuilder.

No behavior change: nothing computes or validates the fields yet.

Testing

• near-primitives unit tests; existing near-chain spice unit tests (placeholder production builds/processes spice blocks).

2/6 of the SPICE light-client stack (#15947). Base: #15962.

[codecov]

Codecov Report

❌ Patch coverage is 76.92308% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.49%. Comparing base (f0e71b0) to head (3f2b832).

Files with missing lines	Patch %	Lines
core/primitives/src/views.rs	57.57%	14 Missing ⚠️
core/primitives/src/test_utils.rs	46.66%	8 Missing ⚠️
core/primitives/src/block_header.rs	95.65%	2 Missing ⚠️

Additional details and impacted files

@@                         Coverage Diff                          @@
##           darioush/spice-lc-merkle-proof-1   #15963      +/-   ##
====================================================================
- Coverage                             72.52%   72.49%   -0.03%     
====================================================================
  Files                                   946      946              
  Lines                                204290   204367      +77     
  Branches                             204290   204367      +77     
====================================================================
+ Hits                                 148154   148159       +5     
- Misses                                51168    51242      +74     
+ Partials                               4968     4966       -2     

Flag	Coverage Δ
pytests-nightly	1.10% <0.00%> (-0.01%)	⬇️
unittests	69.45% <53.84%> (-0.04%)	⬇️
unittests-nightly	69.43% <53.84%> (-0.03%)	⬇️
unittests-spice	66.86% <63.46%> (-0.02%)	⬇️

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

☔ View full report in Codecov by Harness.
📢 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.