perf: ⚡ refactor grain/pid to reduce memory foot-print

[Tochemey]

• Refactor Passivation engine
• Refactor PID struct by trimming it a bit

closes #977

[chatgpt-codex-connector]

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[codecov]

Codecov Report

❌ Patch coverage is 94.59963% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.70%. Comparing base (eff8ff4) to head (ffb1b71).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
actor/passivation_manager.go	92.65%	14 Missing and 9 partials ⚠️
actor/pid.go	94.23%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #978      +/-   ##
==========================================
- Coverage   93.82%   93.70%   -0.13%     
==========================================
  Files         134      136       +2     
  Lines       10434    10813     +379     
==========================================
+ Hits         9790    10132     +342     
- Misses        412      437      +25     
- Partials      232      244      +12     

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