feat(tui): per-session cpu/mem readout + caffeinate flag in sidebar

[mmoscosa]

What

Adds per-session resource visibility to the sidebar, gated behind a single config flag sidebar.show_session_stats (default on):

1. Current cpu/mem inline on every row — a compact <cpu>% <mem> readout (e.g. 5% 1.0G) in the right-hand slot (taking the last-seen-age slot when a stat is available). now = the live sum of every process across every pane in that tmux session.

2. Rolling peak under the focused row — the selected row expands with two lines: cpu <now>% now · <peak>% peak and mem <now> now · <peak> peak. peak = the rolling max over the last 60 proc-poll samples (~2 min).

3. Coffee glyph (Nerd-Font nf-fa-coffee, U+F0F4) beside any session running caffeinate. Scope is correct by construction — only pane process-trees are walked, so an agent-spawned caffeinate (e.g. an editor/agent keep-awake) flags the session but a standalone one started outside tmux does not.

How

• New isolated internal/tui/session_stats.go: computeSessionTotals (reuses the existing aggStats + proc.BuildTree + tmux.GroupBySessions primitives) and SessionStatsModel (rolling-peak ring, pruned when a session disappears). The zero value is usable — no constructor wiring.
• Fed from handleProcDataMsg right next to the existing SetProcLabels call; pushed into the sidebar via SetSessionStats.
• Rendering reuses the existing per-node-height viewport machinery: the inline readout is a normal right-side indicator; nodeHeight becomes cursor-aware (+2 rows for the focused peak lines); the coffee glyph is a themed indicator (IconCaffeine).
• Summed CPU can exceed 100% on multi-core — shown verbatim (matches Activity Monitor).

Tests

go test ./... green. Unit coverage for aggregation, caffeinate detection (positive + negative scope), rolling-peak decay/hold/prune, the config default, the coffee indicator, the inline per-row readout, the focused-row peak lines, and the cursor-aware nodeHeight.

Notes

• Independent of fix: bump gopsutil v3 → v4 to drop go-m1cpu cgo init crash on macOS 26 #56 (the gopsutil-v4 / macOS-26 crash fix) — rebased onto main with no go.mod churn.
• A follow-up PR adds mouse click-to-select to the sidebar; it builds on this one (reuses the cursor-aware nodeHeight + the line→node hit map), so it'll be opened stacked once this lands.