### Motivation

A workflow paywall pre-rendering approach was retired in favor of a
two-surface slide model. Holding every step in the slot table at once
was wasteful (most are off-screen) and the prior animation needed
`snapTo()` + `withFrameNanos()` workarounds to avoid first-frame
flashes.

### Description

Only the current step and the outgoing/incoming step are held in the
slot table during a transition; all other steps are dropped.

Key design:

- `WorkflowPaywallUiState` carries a `pendingTransition` (`fromStepId`,
`direction`, monotonic `id`) set atomically with `currentStepId` in the
ViewModel. The first recomposition after navigation already knows both
surfaces and their initial positions.
- `key(pendingTransition.id)` creates a fresh `Animatable(0f)` during
the composition phase, so frame N's draw immediately sees the correct
offscreen position — no `snapTo()` or `withFrameNanos()` needed.
- `LaunchedEffect` only drives `animateTo(1f)`; it no longer sets up
state.
- `onTransitionComplete(id)` runs after the animation finishes so the
ViewModel can clear `pendingTransition`; a guard on `id` prevents stale
callbacks from clobbering a newer transition.
- Header pinning logic (hero/non-hero, backward) is simplified by
removing the `seenStepId` gap-detection; `pendingTransition` covers both
the "before animation starts" and "animating" cases with the same
branch.
- `navigationDirection` removed from the `PaywallViewModel` interface;
direction lives exclusively in `WorkflowPendingTransition`.

Image cache pre-warming for the off-screen steps is split into a
follow-up PR (RevenueCat#3421) so this PR stays focused on the surface model and
animation.

### Checklist

- [x] Unit tests added (`LoadedWorkflowPaywallHeaderSelectionTest`)
- [ ] If applicable, create follow-up issues for `purchases-ios` and
hybrids

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Introduces new workflow-specific rendering and animation state for
Components paywalls, which can affect navigation, touch handling, and
header/hero layout during transitions. Risk is mostly UI/UX regressions
(flashes, clipping, wrong header selection) rather than data/security
concerns.
> 
> **Overview**
> Adds a dedicated workflow paywall renderer that uses a two-surface
slide transition: `InternalPaywall` now renders `LoadedWorkflowPaywall`
when `workflowState` is present, otherwise it falls back to
`LoadedPaywallComponents`.
> 
> Implements `WorkflowSlideState` to drive horizontal slide animations
keyed by `pendingTransition.id` (keeping only the current +
outgoing/incoming steps in composition) and adds header-selection logic
to keep the correct header visible during hero/non-hero and backward
transitions, covered by new unit tests
(`LoadedWorkflowPaywallHeaderSelectionTest`).
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
88595b3. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>