history.py compare A B — overlay two past cooks on one chart

[ctopherwilliams]

Problem / motivation

history.py list/show/summary let you inspect one cook at a time, but there's no way to compare two side by side. The natural question after a cook — "is this brisket tracking like my last one?" — currently means manually eyeballing two separate trend.py/plot.py outputs.

Proposed solution

Add a compare subcommand: history.py compare <id1> <id2>. Suggested shape:

• Reuse history.sessions()/summarize() to load both cooks.
• Print a side-by-side stats table (duration, peak grill, per-probe start/peak/final, stall time if computed).
• Optionally render an overlay chart — two probe-temp curves on one SVG, time-normalized from each cook's own start (not wall-clock), reusing plot.py's renderer/color scheme where practical.

Alternatives considered

A separate compare.py script instead of a history.py subcommand — rejected in the original roadmap note in favor of keeping it under the existing history.py CLI surface, consistent with list/show/summary.