feat(ui): message inspector + session panel

[sepehr-safari]

Summary

Turns the detail pane into a real message inspector. Selecting a timeline row
(#28) now unpacks the event completely and shows the session it belongs to.

• Normalized fields — id, message id, type, direction, timestamp, and the
  error code/description for CallError.
• Session panel — the session the event correlates into: transaction id,
  status (colored), connector, start/stop, and event count, plus a
  Jump to first event control.
• Payload tree — a model-owned disclosure tree over the std.json.Value
  payload (ui.tree + the ARIA keymap: Up/Down roving focus, Left collapses,
  Right expands, Enter selects). Objects/arrays expand; scalars are leaves
  (strings quoted).
• Raw message — the original OCPP-J array, pretty-printed.

Design notes

• Flat treeitem rows. The tree is a flat, pre-order list of role=.treeitem
  rows with depth encoded as indentation, not physically nested containers. The
  per-row keymap resolves off each row's own expanded state and roving focus
  walks document order, so flatness keeps the full ARIA keymap while cutting each
  row to ~4 widget nodes — the timeline is windowed, but the detail tree is not,
  so node cost matters.
• Collapse-independent ids. Each node's collapse bit is keyed by its
  pre-order rank over the bounded payload. The walk always advances the id
  counter over every in-bounds node (emitting a row only when visible), so a
  collapse bit always names the same node across rebuilds — toggling one node
  never shifts another's identity. State is a fixed-size StaticBitSet on the
  trace, reset when the selection changes.
• Bounded for hostile input. Depth ≤ 6, breadth ≤ 40 per container, ≤ 100
  tracked nodes; past them the tree shows a compact ... N more / ... truncated
  marker. The raw view is capped at 8 KiB. Payloads are untrusted (ADR-0007), so
  the inspector can't be made to blow the widget-node budget or the text layout.

Scope note — timeline scroll on jump

Jump-to-first-event selects and highlights the session's first event and
drives the detail/session panels to it. The literal timeline viewport scroll
is not wired: the zero-config UiApp exposes no model-driven scroll control (the
runtime owns the virtual list's offset, keyed by list id), so revealing an
arbitrary row needs the same runner eject as interactive open — tracked under
#33. For realistic traces the first event is already at the top and visible.

Testing

native test -Dplatform=null — 77 headless tests green (macOS local; CI runs
both OSes). New coverage:

• Payload-tree object/array/scalar shapes, the depth bound (an over-deep chain's
  leaf never renders) and the breadth bound (an over-wide array shows a marker),
  and the total-node bound.
• Collapse toggling hides a container's descendants through the real on_press
  dispatch.
• The session panel reflects the event's session; Jump to first event selects
  the session's first event.
• The accessibility sweep now also covers the enriched detail pane (tree,
  session, raw).

native check --strict clean; native build (ReleaseFast) builds. Living docs
(CURRENT_STATE.md) updated in-branch.

Closes #30