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 67.91%. Comparing base (be23a35) to head (a34b64d).

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     67.94%   67.91%   -0.04%     
  Complexity      739      739              
============================================
  Files          1049     1049              
  Lines         84385    84406      +21     
  Branches      60963    60984      +21     
============================================
- Hits          57336    57321      -15     
- Misses        24692    24723      +31     
- Partials       2357     2362       +5     

Flag	Coverage Δ
rust	70.19% <0.00%> (-0.05%)	⬇️

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

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 10 files with indirect coverage changes

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