Add a widget playground for manual testing and demos

[ashyablok-cs]

Stacked on #676 — based on LYT-1120-widget-dialog-accessible-names, so this diff
shows only the playground. Retarget to develop once #676 merges.

Why

There is no way to look at a pathfora widget without hand-writing an HTML page.
Verifying the #675 accessibility work meant building a throwaway harness twice, and the
same traps cost time both times. This makes that harness a real thing.

playground/ renders any of the 16 valid type × layout combinations. Picking one
from the catalogue loads its config into an editor; change it and render again. The
editor runs as JavaScript in the same shape as the 52 examples in
docs/docs/examples/src, so a snippet from a customer bug report pastes in and runs
verbatim — which is the case I most wanted when reproducing #675.

Run it with yarn run local and open
localhost:8080/playground/. It works on a fresh
clone with no build step, since dist/ is committed.

Widgets render in an iframe

The first working version rendered widgets in the playground page itself. That put a
bottom-left slideout behind the sidebar and a top-fixed bar behind the toolbar, and
there is no corner of the chrome that is reliably out of the way — a toggle only
half-fixes it. So widgets render into playground/stage.html instead. No z-index
contest, no CSS bleed from the playground into the thing being inspected, and a bar
lands exactly where it would on a customer's page. It is the same reason Storybook
frames its canvas.

Two traps it handles

Both cost me real time on #675, and both are invisible when you get them wrong:

• window.PathforaCSS is set before the SDK loads. The constructor reads it and
  runs on evaluation (src/rollup/pathfora.js:141, :150), so a later assignment is
  ignored. Without it the page loads c.lytics.io/static/pathfora.min.css and that
  production CSS wins the cascade over your local build — local CSS edits appear to do
  nothing, with no error. This made me report an animation fix as broken when it was
  fine. Worth knowing independently of this PR: you cannot verify local CSS on any page
  that lets the library load its own stylesheet.
• Stored state is cleared before every render. pathfora.clearAll() resets
  in-memory trackers only — it never touches storage. So a submitted gate stays unlocked
  and impression caps stay spent, across renders and across reloads. Both
  localStorage and sessionStorage are swept (impressions and recommendations write to
  each), by the nine key prefixes in src/rollup/globals/config.js:14-22. Keep stored
  state opts out for anyone deliberately testing impression caps or hideAfterAction.

Two library bugs it surfaces

Found while building the catalogue; neither is fixed here.

• SiteGate/gate is labelled broken in the UI. Its Confirm button does nothing:
  construct-widget-actions.js:272-284 never assigns a widgetAction for
  type: 'sitegate', so the click handler returns early — it never tracks, never writes
  PathforaUnlocked_, never closes. Form with layout: 'gate' is the working
  equivalent. Flagged in the sidebar so nobody debugs it twice.
• Gate layouts get no position. validateWidgetPosition has no case 'gate', so
  choices stays undefined and choices.length throws. It is unreachable today only
  because the call is guarded by if (config.position) and every type defaults
  position to ''.

Also

• Removes test.html. It was an unreferenced, stale version of this: titled "Action
  widget example" while rendering a form gate, and it loads the jstag, which installs the
  production SDK alongside the local dist/ build. Leaving it means two harnesses
  where the worse one is the one people find first.
• Extends the gulp lint glob to playground/**/*.js, which neither that glob nor
  yarn lint previously reached.
• README section under Development.

playground.js is not prettier-formatted. The repo's eslint config sets
wrap-iife: ['error', 'outside'], which wants (function () {}()), and prettier rewrites
that to (function () {})(). eslint is what CI enforces and prettier is not even a
dependency, so eslint wins. The HTML and CSS are prettier-formatted.

Verification

• All 16 combinations render and are visible in the stage; driven programmatically in
  Chrome, not by clicking through.
• No c.lytics.io stylesheet in the stage frame — the local build is what is loaded.
• Submitted a Form/gate, confirmed PathforaUnlocked_ is written, re-rendered and it
  appears again. With Keep stored state ticked it is correctly suppressed, which
  proves the toggle works rather than the sweep being a no-op.
• Editor: edited config takes effect; a syntax error and an invalid type/layout
  combination both surface in the page rather than only the console.
• yarn lint, gulp build, suite at 300 passing — unchanged, and dist/ is
  byte-identical to the parent branch. Nothing under src/rollup/** is touched.

Note yarn lint reports two pre-existing errors on develop
(replace-entity-field.js, expiring-local-storage.js) in files neither branch touches.
CI does not see them because src/**/*.js expands as src/*/*.js under its shell.

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