feat: `Timeline` — passive status-sequence indicator widget

[israel-dryer]

Summary

A new bs.Timeline widget: a passive status-sequence indicator aimed at the
engineering / science / hobbyist audience — for procedure / test / calibration /
acquisition sequences (a "run and watch" status display, not a consumer checkout
wizard). Designed in a planning session; this issue captures the locked design.

Why / value

This fills a real gap. Tabs + Form can fake a click-through wizard, but nothing
renders a live pass/fail/running sequence — exactly what instrument/lab/test UIs
want. The value is in the async/status states, not navigation.

Core decisions

• Name = Timeline, not Stepper. For this audience "stepper" collides with
  stepper motor and biases toward navigation. "Timeline" matches the vertical
  status-sequence look, and the icon+connector rendering is the canonical timeline look.
• Passive. The widget only renders status you set from code. Automation
  (Schedule loop / worker thread / hardware callback / manual clicks) lives outside
  the widget — keeps it model-agnostic. No step-callable execution inside.
• State model (a StepStatus Literal): pending | active | running | pass | fail | skipped | done. running + distinct pass/fail are the point.

Rendering — NO canvas (all icon swaps + composition)

Every concern validated against existing source:

• Nodes = Picture/icon with a live image swap.
  • Numbered nodes use existing glyphs {n}-circle/-fill and {n}-square/-fill
    (verified present 0–9 in assets/icons/glyphmap.json). shape='circle'|'square'
    is a free variant.
  • hollow vs -fill = upcoming vs active; a completed step shows check-circle-fill,
    not its number; fail = x-circle-fill; skip = dash-circle.
  • Numbered only works 0–9 → need a >9 fallback (plain dot).
• Running spinner = Picture animation, fully in-memory, no baked .gif asset
  (fixed pixels can't recolor to accent/theme). Path: render arrow-repeat in the
  node's accent color → PIL-rotate to ~8–12 frames → encode to in-memory animated
  WebP/GIF (BytesIO) → Image.from_bytes → Picture(loop=True) auto-plays it.
  Every node being a Picture means running is the same image swap as any state.
• Connectors = static Divider (already has thickness/accent/orient/length).
  Uniform muted line; all state lives in the nodes. No live-setter needed.
• Labels / per-step result lines = Label / Label(textsignal=...)
  (e.g. R = 10.02 kΩ ✓). Vertical orientation shines here.

Proposed API

tl = bs.Timeline(
    orientation="vertical",   # "vertical" (default) | "horizontal"
    accent="primary",
    numbered=False,           # status-glyph nodes vs numbered badges
    shape="circle",           # "circle" | "square"
    clickable=False,
    density=None,
    steps=None,               # declarative, via the option data bag (normalize_option)
)

step = tl.add_step(
    "Calibrate sensor",
    icon="target",            # optional; else status glyph
    description="Zero the offset",
    status="pending",
    on_select=handler,        # optional click handler
)

# live Step handle (like TreeNode / Image handle)
step.status  = "running"
step.status  = "pass"
step.message = "R = 10.02 kΩ"
step.disabled = True

tl.current = 2                # live prop; repaints + emits
tl.on_step_select(lambda e: ...)   # → StepEvent(index, step)

add_step returns a handle, not a context manager (a step is data, not a content
container).

Prerequisites

• None hard.
• Optional convenience: an Image.from_frames([imgs], duration=) helper to hide the
  BytesIO encode for the spinner (independently useful for any procedural animation).

Out of scope (separate, later)

• Wizard — a separate orchestration widget composing Timeline (horizontal) +
  PageStack + a Back/Next/Finish bar + linear/validation gating bound to Form.valid.
  Pure assembly; decide widget-vs-recipe after Timeline exists. Mirrors the
  PageStack ← AppShell/Tabs layering.

Build order

1. Timeline indicator (this issue).
2. Wizard follow-on (separate issue, once the indicator's API is proven).

[israel-dryer]

Follow-on orchestration widget now tracked in #312 (Wizard composes this Timeline). Build order: #311 first, then #312.