feat(consensus): Add metadata and partition handles for MuxPlane

[krishvishal]

The previous code accessed planes through positional tuple indexing (planes.0, planes.1.0), which was fragile and opaque. The new handle traits give each plane a semantic name, making call sites self-documenting and won't create problems when changes are made to plane ordering.

• Introduce MetadataHandle and PartitionsHandle traits that provide typed, ergonomic accessors for the heterogeneous tuple planes inside
  MuxPlane
• Implement these traits for the underlying tuple types (M, Tail) and (M, (P, ())), and delegate through MuxPlane

[codecov]

Codecov Report

❌ Patch coverage is 0% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.96%. Comparing base (118ab6a) to head (affb153).

Files with missing lines	Patch %	Lines
core/consensus/src/plane_mux.rs	0.00%	24 Missing ⚠️
core/shard/src/lib.rs	0.00%	10 Missing ⚠️

❌ Your patch check has failed because the patch coverage (0.00%) is below the target coverage (50.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2855      +/-   ##
============================================
+ Coverage     67.91%   68.96%   +1.05%     
+ Complexity      739      656      -83     
============================================
  Files          1049      858     -191     
  Lines         84373    69206   -15167     
  Branches      60951    60683     -268     
============================================
- Hits          57299    47731    -9568     
+ Misses        24710    19127    -5583     
+ Partials       2364     2348      -16     

Flag	Coverage Δ
go	?
node	?
python	?
rust	70.19% <0.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
core/shard/src/lib.rs	0.00% <0.00%> (ø)
core/consensus/src/plane_mux.rs	0.00% <0.00%> (ø)

... and 207 files with indirect coverage changes

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