feat: configurable numeric precision (per-widget, type-aware, optional global)

[zachthedev]

Problem

The numeric formatters render a fixed decimal count, so whole values carry a
trailing .0: 1.0M, 512.0k, 100.0%, 50.0 t/s. Blanket-stripping it
(the original #456) was rejected: the fixed width stops the field from snapping
as a value crosses a whole unit (1M -> 1.1M -> ...), which Claude Code
itself preserves (6.0k). The fix is to make precision configurable, not to
change the default.

Proposal

A per-widget number-format setting, type-aware, with an optional global.
Default output is unchanged.

Format: numberFormat = { style?, decimals? }

• style: precise (keep trailing zeros, today: 1.0M) | compact (drop
  trailing zeros, keep real fractions: 1M, 1.1M) | whole (no decimals: 1M)
• decimals: max places before style applies (precise + 2 -> 1.00M)

Type-aware defaults

Each numeric widget has a number kind with its own default, so one setting
does the right thing per type:

kind	widgets	default
token	tokens in/out/cached/total, context length, context-bar count, compaction reclaimed, cache read/write	precise, 1 (1.0M)
speed	input/output/total speed	precise, 1 (50.0 t/s)
percent	context %, cache hit %, usage/utilization/reset sliders	precise, 1 (84.5%)
memory	free memory	current (G: 1, M/K: 0)
cost	session cost	precise, 2 ($1.20)

So tokens can be xxx.x while money stays xxx.xx: cost defaults to 2 and a
token-oriented change won't pull it to 1.

Precedence

effective = global[kind] (if set) -> widget.numberFormat (if set) -> kind default

Per-widget is the primary control. The global is optional and off by default;
when set it wins, the same way overrideForegroundColor / globalBold already
do. With nothing set, every widget renders exactly as today.

Scope

formatTokens + consumers, formatSpeed, FreeMemory, the toFixed(1)%
percentage sites, SessionCost.

Implementation notes

• numberFormat is an optional top-level WidgetItem field (parallel to
  dim/bold); global is an optional per-kind Settings field. Optional, so
  no settings-version migration; absent = current behavior.
• One resolveNumberFormat(kind, item, settings) choke point; pure formatters
  take the resolved { style, decimals }; shared helpers take it as a param.
  Precision is resolved at render time (it can't be a post-render pass like
  color/bold).
• TUI: per-widget affordance in the editor (like (d) for dim) + a global
  entry in the overrides menu.

Non-goals

• No change to default output. Cost stays 2-decimal by default.