feat(runtime): context propagation API

[eliottness]

https://docs.google.com/document/d/18ZLadxvljFySnJ-3cFhTfljKHayXRClJ9GmPXoDWMRU/edit?tab=t.0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.98%. Comparing base (87ba6d2) to head (1b8589d).

❗ There is a different number of reports uploaded between BASE (87ba6d2) and HEAD (1b8589d). Click for more details.

HEAD has 12 uploads less than BASE

Flag	BASE (87ba6d2)	HEAD (1b8589d)
Windows	1	0
DRIVER	1	0
integration	1	0
ARM64	1	0
go1.25rc2	1	0
go1.23	1	0
generators	1	0
GOFLAGS	1	0
Linux	1	0
TOOLEXEC	1	0
go1.24	1	0
unit	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #664       +/-   ##
===========================================
- Coverage   68.71%   47.98%   -20.73%     
===========================================
  Files         113      110        -3     
  Lines        7926     7498      -428     
===========================================
- Hits         5446     3598     -1848     
- Misses       1939     3482     +1543     
+ Partials      541      418      -123     

Components	Coverage Δ
Generators	∅ <ø> (∅)
Instruments	∅ <ø> (∅)
Go Driver	44.65% <ø> (-31.87%)	⬇️
Toolexec Driver	68.52% <ø> (-2.31%)	⬇️
Aspects	57.15% <ø> (-17.17%)	⬇️
Injector	57.85% <ø> (-17.02%)	⬇️
Job Server	49.05% <ø> (-20.18%)	⬇️
Other	47.98% <ø> (-20.73%)	⬇️

see 56 files with indirect coverage changes

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