feat(spice): per-block batch light-client variant (experiment)

[darioush]

Experimental per-block "batch" (fan-out) variant of the SPICE light-client execution anchor, built head-to-head against the shipped global-accumulator stack (#15962–#15967) to settle whether global was the right call. Based on PR3 (darioush/spice-lc-roots-reader-3), so this diff is the fan-out divergence only and lines up against global's PR4–6.

Instead of one growing fork-safe accumulator anchored at the head, each block commits a small Merkle root (reusing the existing certified_block_merkle_root field) over only the blocks it newly certifies. A proof anchors on the certifying block C via a flat H -> C lookup (the single new CertifiedByBlock column), then anchors C into the head's block_merkle_root with today's consensus block proof. Validation is self-contained: recompute the batch root from the block's own statements and compare, with the index re-pointed to follow the canonical chain on reorg.

Trade vs global: ~31% less production code across 4 fewer files, one DB column instead of three and no new persisted borsh types, at the cost of a permanent 6-field proof (vs 4) and a verification asymmetry (the merklize batch hop hashes its leaf). The comparison writeup recommends keeping global, since the proof cost is paid forever and is consensus-facing while the production-state complexity is paid once behind an internal interface.

Experiment only; not intended to merge.

[codecov]

Codecov Report

❌ Patch coverage is 85.82524% with 73 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.52%. Comparing base (ae46722) to head (db58834).

Files with missing lines	Patch %	Lines
chain/chain/src/spice/core.rs	83.44%	8 Missing and 17 partials ⚠️
chain/chain/src/chain.rs	64.86%	6 Missing and 7 partials ⚠️
chain/client/src/view_client_actor.rs	81.69%	5 Missing and 8 partials ⚠️
core/store/src/merkle_proof.rs	89.83%	3 Missing and 3 partials ⚠️
core/primitives/src/test_utils.rs	73.33%	4 Missing ⚠️
chain/chain/src/store/mod.rs	90.62%	1 Missing and 2 partials ⚠️
chain/chain/src/chain_update.rs	75.00%	1 Missing and 1 partial ⚠️
chain/client/src/client.rs	75.00%	0 Missing and 2 partials ⚠️
chain/jsonrpc/src/lib.rs	50.00%	2 Missing ⚠️
core/primitives/src/block_header.rs	95.65%	2 Missing ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15968      +/-   ##
==========================================
+ Coverage   72.49%   72.52%   +0.02%     
==========================================
  Files         946      946              
  Lines      204277   204704     +427     
  Branches   204277   204704     +427     
==========================================
+ Hits       148094   148461     +367     
- Misses      51215    51240      +25     
- Partials     4968     5003      +35     

Flag	Coverage Δ
pytests-nightly	1.10% <0.00%> (-0.01%)	⬇️
unittests	69.34% <25.93%> (-0.13%)	⬇️
unittests-nightly	69.32% <25.93%> (-0.13%)	⬇️
unittests-spice	66.93% <83.26%> (+0.06%)	⬆️

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