feat(tabs,header): group tabs by state and add header stats styles (#426)

Two directions from Discussion #426, each a per-device setting and the
new default.

Tab Grouping (tabGrouping: 'state' | 'none', default 'state', option C):
tabs are grouped needs you (red, plus failed sessions), waiting (yellow),
working and idle (also ended, exited panes and web tabs), most urgent on
top. The desktop header strip draws a row per group with its label and
count in a left gutter; the flat vertical rail and the sidebar draw a
section per group; tablets keep their scrolling row with inline dividers;
phones keep the chip row in group order without headings. Classification
is the home screens' own (_mobileOverviewState/_mobileOverviewExit), the
fold into four groups is pure in CodemanTabTriage (constants.js). It is
flex `order` plus aria-hidden heading/break elements reconciled in place
after both render paths, never a DOM reorder, so Alt+N, the keyboard walk
and drag keep reading tab order; a drop is refused across groups. Named
groups in the vertical rail take precedence.

Header Stats Style (headerStatsStyle: 'classic' | 'compact' | 'tiles',
default 'tiles', option G): tiles give WS, CPU, MEM and each plan window
a label-over-value tile with a bar underneath; compact is one WS/CPU/MEM
pill with sparklines plus a plan-ring pill; classic is the header as
before. Desktop only (classic below 768px and in solo windows). The
clustered styles move #connectionIndicator into #headerSystemStats and
#planUsageChip after it, and classic moves them back to comment anchors;
WS stays out of a hidden System Stats pill. The extra parts are always
rendered and hidden by default in CSS, so classic is unchanged.

Tests: test/tab-triage.test.ts, test/header-stats-style.test.ts; three
source pins in test/tab-rail-order.test.ts follow renamed lines.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/architecture-invariants.md

@@ -865,6 +865,24 @@ Tests: `test/terminal-touch-tap.test.ts`.
 
 ⚠️ Mobile no longer hoists the active session to the front of the strip: that reordering ran on full renders only, so tab order flipped depending on which render path fired, and it renumbered the Alt+N badges. Scroll-into-view replaces it; do not reintroduce it.
 
+### Tab grouping by state
+
+**Tab grouping by state** (`tabGrouping`, per-device, default `state`; Discussion #426 option C): the tab list is split into four groups, most urgent first: **needs you** (a permission or question dialog; a failed session joins it), **waiting** (idle prompt pending), **working**, **idle** (also ended sessions, an agent that exited inside a live pane, and web tabs). The desktop header strip draws a row per group with its label and count in a left gutter; the flat vertical rail and the sidebar draw a section per group; the tablet strip (600-767px) keeps its scrolling row with the headings as inline dividers; phones keep their chip row in group order with the headings hidden (mobile.css). Classification is `_mobileOverviewState()` + `_mobileOverviewExit()`, the home screens' own, and the fold into four groups is pure in `CodemanTabTriage` (constants.js, `computeTabTriageLayout()`).
+
+⚠️ **It is the flex `order` property, never a DOM reorder**, exactly like the sorted rail: `#sessionTabs` stays in `sessionOrder`, so the Alt+N badges, drag, the keyboard walk (which already sorts by COMPUTED order) and the sidebar filter keep reading the list they always read. Each group owns a band of `TAB_TRIAGE_STRIDE` order values: its heading, its rows, its web tabs, then the row break that ends the header line. The headings and breaks are `aria-hidden` direct children of `#sessionTabs`, reconciled in place by `_syncTabTriageChrome()` after BOTH render paths, because a state change is an incremental pass (no tab added or removed) and can still empty a group or fill a new one. An unchanged pass writes nothing.
+
+⚠️ Inside a group a header row keeps TAB order, so the strip only moves a tab when its state changes; a sorted rail ranks each section the way it ranks the flat rail (`_tabRailSortOrder()` feeds `pos`). Drag stays on in the strip, but `_isTabDropAcrossTriageGroups()` refuses a drop on a tab in another band: the dragged tab would stay in its own group and land where the user did not put it.
+
+⚠️ **Named groups win**: in the vertical rail with owner tab layouts (`_projectTabGroups()` non-null) triage is off and the grouped tree renders as before. With `tabGrouping: 'none'` nothing is left behind: no headings, no breaks, no inline order, no `tabs-triage` class. A stale cached mobile-overview.js degrades to the flat strip. The header strip only wraps into rows on desktop, where `updateTabOverflowMode()` forces `tabs-auto-wrap` while `tabs-triage` is on; the breaks only display in a wrapping strip, since a `flex-basis: 100%` break in a nowrap scroller would steal width. Tests: `test/tab-triage.test.ts`.
+
+### Header stats styles
+
+**Header stats styles** (`headerStatsStyle`, per-device, desktop only, default `tiles`; Discussion #426 option G): how the WS readout, CPU, MEM and the plan-usage windows are drawn. `tiles` gives each its own label-over-value tile with a bar along the bottom edge; `compact` is one `WS · CPU · MEM` pill with a 4-sample sparkline per stat plus a pill with a ring per plan window; `classic` is the bars and `5H · 7D` chip as before. `data-header-stats` on `<html>` (pre-paint, then `applyHeaderStatsStyle()` from `applyHeaderVisibilitySettings()`) drives every rule; narrower than 768px and in a solo window it resolves to `classic`, because the stats are hidden there anyway and the moves below would otherwise reach the phone header.
+
+⚠️ The clustered styles need the three elements contiguous, and the template keeps the classic order, so `applyHeaderStatsStyle()` MOVES `#connectionIndicator` into `#headerSystemStats` (first child) and `#planUsageChip` right after it, and `classic` moves them back to comment anchors it left at the template positions. Ids are unchanged, so every writer still finds them. ⚠️ The indicator joins the pill only while System Stats is shown: the pill is hidden with `display: none`, and the WS readout must not disappear with it.
+
+⚠️ The extra parts (`.stat-spark`, `.connection-tile-label`/`-value`, `.pu-ring`, `.pu-meter`) are always rendered and hidden by default in styles.css, which is what keeps `classic` looking exactly as before. The tile words come from `_connectionTileWords()`, DERIVED from the connection descriptor rather than added to it, because `test/connection-indicator.test.ts` pins the descriptor's exact shape. Plan rings and meters clamp their fill to 0-100 while the label keeps the real number. Tests: `test/header-stats-style.test.ts`.
+
 ### Phone overview home screen
 
 **Phone overview home screen** (`mobile-overview.js`, phones only, per-device `mobileOverviewEnabled`, default ON): under 600px the "C" logo shows a session overview (NEEDS YOU / CURRENT SESSIONS / PAST SESSIONS) instead of the welcome overlay; tablet and desktop are unchanged. The branch lives in `showWelcome()`/`hideWelcome()` (terminal-ui.js) behind `shouldUseMobileOverview()`, which is **width-driven** (`getDeviceType() === 'mobile'`) because this is a layout decision, unlike the settings namespace which stays handheld-based.