feat(web-explorer): Sen Web Explorer + npm license attribution

[EnriqueParodi]

Browser-based UI for inspecting and interacting with a running Sen process.
React + TypeScript on top of @sen/client; served by jsonrpc's static-file
server so the browser only ever talks to one origin (the JSON-RPC endpoint
that also carries the live data).

What you can do with it:

• Browse the live topology (sessions, buses, objects matched by named
  queries). Author and reuse interests across sessions; queries persist.
• Inspect any object's properties with type-aware rendering for structs,
  sequences, variants, and units. Edit writable properties in place.
• Invoke methods with typed argument editors.
• Watch events into a unified bottom drawer plus per-class tables with
  resizable columns, source filtering, and a live row-arrival highlight.
• Plot numeric leaves on a multi-series board: drag series between panels,
  share a cursor across panels, brush-zoom, pause/resume, scrub through
  a retention-bounded history. Pop the board out to a second monitor.
• Pin objects to a Cockpit / Overview workspace for at-a-glance monitoring
  across instances, with a class-grouped Overview to navigate dense
  topologies.
• Save layouts: panel arrangement, column widths, retention windows,
  pinned set, open queries all round-trip through localStorage.
• Light + dark themes.

Layer model:

• Frontend is a thin reactive skin over @sen/client; no Sen-specific
  schema hard-coded anywhere in the UI. Every type, property, and unit
  is discovered at runtime.
• C++ component subscribes to jsonrpc's StaticFileServer control bus
  and registers a baked index.html bundle at /explorer.
• gridstack v12 for the resizable grid (PlotsView + OverviewWorkspace);
  uPlot for time series; react-virtuoso for the event tables.
• One bundled index.html, no runtime CDN fetch, no telemetry.

Correctness commitments:

• WatchCollectors mount event-watch wire subscriptions independently of
  the Events tab; an event watch with no Events tab open now sees its
  values, not "--".
• The three useMemo-for-ref-mutation sites in u_plot_chart became
  sentinel-refs (React 19 drops memoised values for offscreen subtrees).
• Plot panel layout persistence carries a schema version envelope.
• Panel ids are crypto.randomUUID, not monotonic counters.
• defaultUrl trusts same-origin; the prior port allowlist would have
  blocked legitimate https deployments behind a reverse proxy.
• Object handle swaps on reconnect skip the gap sentinel; the prior
  behaviour briefly showed "no value" across every reconnected widget.
• gridstack cross-doc monkey-patch removed (was patching the private
  _mouseDown / _mouseMove / _mouseUp / _keyEvent for popup-window
  portals); popouts now use layout-lock for portable resize.
• Modal focus-management effect no longer depends on onClose; the prior
  deps would re-fire on every parent render and steal focus from any
  input inside the modal (real bug, caught during user testing).
• ObjectEventCollector eventNames sealed readonly; the prior for ... of
  iterated over a mutating array on a parallel resubscribe path.
• gridstack pinned to an exact version + fail-loud private-method assert;
  if a minor bump silently changes private internals the explorer fails
  fast instead of silently misrendering.

Plus extensive polish across the plot board (rAF-batched render, per-line
cursor labels, brush-zoom integrated with uPlot's native pattern, panel
drag-between, axis-per-unit, retention slider bound to window cap) and
a CLI shortcut: sen web-explorer bakes a yaml preset (ether + jsonrpc +
webexplorer) into cli_run and opens the browser at the right URL.

Plus an npm license attribution helper (sen_internal_bake_npm_licenses)
that the web explorer calls to bake its runtime dependency licenses into
the install bundle; reusable by any future npm-built frontend.

Ships with the getting-started guides:

Closes the gap between what the tree can do and what a newcomer can find:

• install.md: 'What the build needs' -- toolchain-via-Conan (incl. Node.js
  arriving as a tool requirement, not a host install), the Conan Center +
  npm registry network requirement, first-build time, how to skip the
  browser stack, Windows caveat.
• docs/getting_started/testing.md (new, in nav): enabling tests, the
  run_tests lanes (flaky tier with until-pass:5, junitparser merge), a
  table of suites that are environment-sensitive by design (previously
  tribal knowledge), and the Debug lane where SEN_DEBUG_ASSERT and the
  invariant checks actually compile.
• README How to Build: short pointers to the above and to the Web
  Explorer showcase; the showcase itself stays documented with its
  component (docs/components/web_explorer.md), which now links back to
  the install guide for getting a built tree.