[macros] add support for on_start/on_end to select_loop!

[andresilva]

This PR extends the select_loop! macro with optional on_start and on_end blocks, enabling cleaner patterns for code that needs to run at the beginning or end of each loop iteration.

The on_start block executes before the select, allowing variable initialization that's visible to all arms. The on_end block runs after any non-shutdown arm completes but is skipped when shutdown triggers, which is useful for cleanup or post-processing logic. Both blocks share lexical scope with the select arms.

This allows migrating some more places to use the select_loop! macro.

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[andresilva]

For the places where I updated the code to use select_loop! this is much easier to review by using difftastic locally (although this will ignore whitespace changes).

[codecov]

Codecov Report

❌ Patch coverage is 86.50307% with 132 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.62%. Comparing base (82547f4) to head (a59d60b).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
consensus/src/marshal/actor.rs	87.40%	32 Missing ⚠️
consensus/src/aggregation/engine.rs	65.43%	28 Missing ⚠️
consensus/src/ordered_broadcast/engine.rs	70.83%	28 Missing ⚠️
consensus/src/simplex/actors/batcher/actor.rs	93.27%	15 Missing ⚠️
consensus/src/simplex/actors/voter/actor.rs	90.34%	14 Missing ⚠️
broadcast/src/buffered/engine.rs	78.12%	7 Missing ⚠️
resolver/src/p2p/engine.rs	95.37%	5 Missing ⚠️
macros/src/lib.rs	92.30%	3 Missing ⚠️

@@           Coverage Diff            @@
##             main    #2671    +/-   ##
========================================
  Coverage   92.62%   92.62%            
========================================
  Files         357      357            
  Lines      102996   103136   +140     
========================================
+ Hits        95396    95534   +138     
- Misses       7600     7602     +2     

Files with missing lines	Coverage Δ
macros/src/lib.rs	78.20% <92.30%> (+2.82%)	⬆️
resolver/src/p2p/engine.rs	96.65% <95.37%> (-0.02%)	⬇️
broadcast/src/buffered/engine.rs	94.01% <78.12%> (+0.40%)	⬆️
consensus/src/simplex/actors/voter/actor.rs	95.67% <90.34%> (-0.02%)	⬇️
consensus/src/simplex/actors/batcher/actor.rs	94.58% <93.27%> (-0.02%)	⬇️
consensus/src/aggregation/engine.rs	85.17% <65.43%> (+0.17%)	⬆️
consensus/src/ordered_broadcast/engine.rs	86.24% <70.83%> (+0.16%)	⬆️
consensus/src/marshal/actor.rs	92.63% <87.40%> (-0.15%)	⬇️

... and 7 files with indirect coverage changes

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