Add notebook profiler with live Rich tree display

[1kaiser]

What does this PR do?

Adds a notebook profiling subsystem to papermill — per-cell timing grouped by
markdown section headings, with an optional live Rich tree display that replaces
the tqdm progress bar during execution.

Motivation

Papermill already records start_time, end_time, and duration in every
cell's metadata.papermill block. This PR surfaces that data in two ways:

1. papermill-profile <executed.ipynb> — reads any executed notebook and
   emits a structured JSON report showing which sections and cells took the
   most time, what output types were produced, and where the bottleneck is.

2. papermill notebook.ipynb out.ipynb --live-tree — shows a live Rich
   tree during execution instead of the tqdm bar:

notebook.ipynb  6/10 cells  60%
├── ✓  Section 1                           0.1s
│   ├── ✓  [1] import numpy as np          0.1s
│   └── ✓  [2] data = np.zeros((n, m))     0.0s
├── ⟳  Section 2                           4.2s…
│   └── ⟳  [4] model.fit(X, y)             4.2s…
└── ·  Section 3                           pending

Section labels are auto-generated sequential numbers (Section 1,
Sub-section 1.2, …) derived from markdown heading depth — no raw notebook
source code appears in the display.

Changes

File	Change
papermill/profile.py	New — build_sections(), build_profile(), profile_notebook()
papermill/live_tree.py	New — LiveTreeDisplay (optional Rich dependency)
papermill/tests/test_profile.py	New — 20 unit tests
papermill/execute.py	live_tree=False parameter
papermill/engines.py	live_display= on NotebookExecutionManager; hooks into cell_start/complete/exception
papermill/cli.py	--live-tree flag; papermill profile subcommand
papermill/__init__.py	Export profile_notebook, build_profile, build_sections
pyproject.toml	optional-dependencies.rich = ["rich>=13.0"]

New entry points

# Live tree during execution (requires pip install 'papermill[rich]')
papermill notebook.ipynb out.ipynb --live-tree

# Profile an already-executed notebook
papermill-profile executed.ipynb

# Python API
from papermill import profile_notebook
profile = profile_notebook("out.ipynb")
print(profile["bottleneck"])
# {'cell_index': 5, 'section': 'Sub-section 2.1', 'duration_s': 18.0, 'pct_of_total': 42.8}

Test plan

• pytest papermill/tests/test_profile.py — 20 tests, all passing
• papermill notebook.ipynb out.ipynb --live-tree --kernel python3 — tree renders live
• papermill-profile out.ipynb — summary printed, .profile.json written
• rich not installed → graceful fallback to tqdm with warning

🤖 Generated with Claude Code