Bump version to 2.2.0

[typeless]

Version bump for the 2.2.0 release: the container-as-allocator campaign (#101, #102, #103, #104), the Arena32 quadratic fix, build-correctness fixes (output-less command scheduling, xwin header deps, show-exporter filtering), and the CI metrics infrastructure (#105, #106).

🤖 Generated with Claude Code

https://claude.ai/code/session_01U5KUoGDhJEZYoiK9CkeXtz

[github-actions]

PR metrics

Performance (gcc example, Linux)

Workload	Instructions	CPU time	Page faults	D1 miss	LL miss	Wall	Peak RSS
parse	n/a	0.44 s	14.1 k	1.1%	0.1%	0.44 s	32.9 MB (+0.1MB)
dry-run	n/a	0.65 s	18.6 k	1.6%	0.1%	0.689 s	49.7 MB (-0.1MB)

Deterministic signals: page faults, peak RSS, and the cachegrind D1/LL miss rates (simulated cache, exact across runs). CPU time is user+sys from time(1); instructions read n/a on GitHub-hosted runners (virtualized, no PMU).

Test coverage (lines)

Overall	Median file	Min file	Max file
85.1%	94.3%	0.0% include/pup/core/metrics.hpp	100.0% include/pup/core/arena.hpp

93 files · 13697/16104 lines covered

Deltas vs main@706ca39c5.

Updated for b799304