docs: add trait overview table and extension guide for unified pipeline

[nh13]

Summary

• Update base.rs module docs with a comprehensive trait overview table organized by category (lifecycle, step state, worker state, data measurement), including "implement when..." guidance for each trait and a hierarchy diagram
• Add extension guide to mod.rs explaining the steps to add a new pipeline type
• Docs only — no code changes

Test plan

• Verify cargo doc --document-private-items --no-deps -p fgumi builds without new warnings
• Review rendered trait tables in rustdoc output

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.18%. Comparing base (ea6e3fc) to head (a7b8bf6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #222   +/-   ##
=======================================
  Coverage   88.18%   88.18%           
=======================================
  Files         113      113           
  Lines       53388    53388           
=======================================
+ Hits        47080    47081    +1     
+ Misses       6308     6307    -1     

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 31ba4e6f-3b4e-4775-9d18-0d4913d1b85d

📥 Commits

Reviewing files that changed from the base of the PR and between ea6e3fc and a7b8bf6.

📒 Files selected for processing (2)

• src/lib/unified_pipeline/base.rs
• src/lib/unified_pipeline/mod.rs

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📝 Walkthrough

Walkthrough

Module-level documentation updates across the unified pipeline crate. base.rs documentation reorganized to describe the trait hierarchy (lifecycle/monitoring, shared step state access, per-worker state traits, measurement traits, and generic worker loop), along with updated references to shared step functions including shared_try_step_process, shared_try_step_serialize, and shared_try_step_write_new. mod.rs extended with a new "Adding a New Pipeline Type" section detailing the checklist of traits to implement (pipeline state traits, worker state traits), integration patterns via StepContext, and guidance on reusing shared step functions. No public API changes.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly describes the main changes: adding trait overview documentation and an extension guide for the unified pipeline.
Description check	✅ Passed	The description is directly related to the changeset, covering the documentation updates to both base.rs and mod.rs with clear intent and test plan.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch nh/unified-pipeline-docs

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