Feature: Hierarchical Process Tree View (pstree-style foldable hierarchy in Terminal UI)

[OrDinaD]

Problem Description / Motivation

The current Terminal UI in client/src/ui/terminal_ui.cpp renders processes strictly as a flat list sorted by CPU, memory, PID, or thread count. While this works well for identifying individual resource-heavy tasks, it makes it difficult to diagnose multi-process application workloads (e.g. systemd service trees, container process trees, fork-exec workers like nginx/gunicorn/postgres, or parallel build jobs). Operators need the ability to view parent-child hierarchies (pstree-style) with interactive fold/unfold capability to quickly isolate rogue process subtrees and inspect aggregated resource usage.

Technical Details & Affected Components

• Procfs Parsing & Data Models:
  • In server/src/collectors/linux_samples.cpp (ParseProcess), /proc/[pid]/stat is parsed, but the parent process ID (ppid, field 4 in stat) is currently omitted.
  • In common/include/common/models/process_metrics.hpp and common/src/common/models/process_metrics.cpp, ProcessInfo lacks a ppid field.
  • In common/src/protocol.cpp, Decode() validates JSON process records without ppid. Adding ppid should be backward compatible or versioned cleanly.
• Client Terminal UI & State:
  • In client/include/client/ui/terminal_ui.hpp and client/src/ui/terminal_ui.cpp:
    • ViewState only tracks sort and flat scroll.
    • Processes() sorts the flat vector and prints rows directly.
  • In client/src/app/client_app.cpp:
    • Key dispatch handles j, k, c, m, p, t, but lacks a keybinding to toggle tree view (e.g., F5 or T) or toggle folding on a selected node (e.g., Space or Enter, + / -).

Proposed Implementation Strategy

1. Model & Serialization:
   • Parse field 4 (ppid) in server/src/collectors/linux_samples.cpp during /proc/[pid]/stat parsing into ProcessSample.
   • Add ProcessID ppid{} to ProcessInfo, and serialize it to/from JSON in ProcessInfo::ToJson() and ProcessInfo::FromJson().
   • Update protocol validator in common/src/protocol.cpp to accept optional/required ppid.
2. Process Tree Data Structure:
   • Construct a directed tree/forest (std::unordered_map<ProcessID, std::vector<ProcessID>>) rooted at PID 1 (or orphaned/root parents) in client/src/ui/terminal_ui.cpp when in tree mode.
   • Support collapsible branches via a set of collapsed PIDs: std::unordered_set<ProcessID> collapsed_nodes.
   • Implement depth-first traversal generating tree branch drawing glyphs (e.g., ├─, └─, │ , [+], [-]).
3. Interactive Navigation:
   • Add cursor selection tracking in ViewState (selected PID row) in client_app.cpp.
   • Support toggling tree mode with T or F5.
   • Support expanding/collapsing nodes with Space, +, -, or arrow keys (Left collapses, Right expands).
   • Show cumulative subtree metrics (summed RSS and CPU%) on collapsed parent nodes.

Acceptance Criteria

• ppid is reliably collected on Linux and transmitted via snapshot protocol without breaking backwards compatibility.
• Terminal UI supports switching between flat list and hierarchical tree mode using hotkey (T).
• Tree view renders parent-child relationships with clean Unicode glyphs and indentation corresponding to process depth.
• Nodes can be collapsed and expanded interactively, with collapsed parent displaying aggregate subtree resource consumption.
• Test coverage added for tree hierarchy building, orphan process handling, cycle avoidance, and JSON deserialization.