refactor: move shared retry and concurrency handling

[jahvon]

Simplify the serial and parallel runners by moving retry handling and work scheduling into the engine package. This includes updates to the runner function signatures and executable runner tests

[codecov]

Codecov Report

Attention: Patch coverage is 74.86339% with 46 lines in your changes missing coverage. Please review.

Project coverage is 37.50%. Comparing base (6f324a0) to head (27f9df8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/runner/engine/engine.go	77.88%	23 Missing ⚠️
internal/runner/serial/serial.go	42.10%	9 Missing and 2 partials ⚠️
internal/runner/parallel/parallel.go	70.58%	3 Missing and 2 partials ⚠️
internal/runner/engine/retry/retry.go	90.90%	2 Missing and 1 partial ⚠️
cmd/internal/exec.go	50.00%	0 Missing and 1 partial ⚠️
internal/runner/launch/launch.go	0.00%	1 Missing ⚠️
internal/runner/render/render.go	0.00%	1 Missing ⚠️
internal/templates/templates.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #195      +/-   ##
==========================================
+ Coverage   35.39%   37.50%   +2.10%     
==========================================
  Files          76       76              
  Lines        6619     6348     -271     
==========================================
+ Hits         2343     2381      +38     
+ Misses       3840     3534     -306     
+ Partials      436      433       -3     

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