feat: document BYOS and multi-action button chaining

[kherembourg]

Summary

Adds the missing coverage for two SDK concepts that were never referenced anywhere in the plugin:

• Bring Your Own Screen (BYOS) — new concept reference (references/concepts/byos.md) covering when to use it (native login step in a Flow, A/B vs. existing paywall, reordering onboarding), Console config (Screen ID + connections), iOS implementation (PLYCustomScreenViewControllerDelegate + SwiftUI PLYCustomScreenViewDelegate), Android implementation (PLYCustomScreenProvider + PLYCustomScreen.View / .Fragment), the executeConnection(...) / execute(connection) handover contract (Flow + standalone), Purchasely.synchronize() requirement for in-screen purchases, analytics behaviour (PRESENTATION_DISPLAYED only, in-screen interactions instrumented by the app), and the anti-patterns BYOS replaces (custom VC over Purchasely, Purchasely.close() then push, skipping display()). iOS + Android only, SDK ≥ 5.6.0.
• Multi-action button chaining — new section in paywall-actions.md documenting how a Composer button can carry several actions (purchase + open_screen / + open_placement / + deeplink / + close, login + purchase, …), the default when no second action is configured (close in Full mode, stay open in Observer mode), the fact that the interceptor sees each action separately, and that proceed(false) short-circuits the chain.

Cross-linked from:

• concepts/README.md index + "When to load" table
• sdk-expert agent (new response rule #13, dedicated BYOS routing + SDK 5.6 / platform gating)
• integrate skill (Step 9 — Beyond the Basics)
• review skill (new section 3.12 BYOS with 8 checkpoints, plus a new anti-pattern checkpoint in 3.3 flagging interceptors that try to override the post-purchase flow instead of using a second Composer action)
• CHANGELOG.md under [Unreleased]

Test plan

• /purchasely:question "Comment afficher un écran de login dans un Flow ?" → must load byos.md and recommend BYOS over presenting a custom VC over the Purchasely controller
• /purchasely:question "Que se passe-t-il après un purchase si je n'ai pas configuré de 2e action ?" → must cite the Full vs. Observer default
• /purchasely:integrate walkthrough surfaces BYOS + chained actions in Step 9 for relevant projects
• /purchasely:review on a project with setCustomScreenProvider triggers the new 3.12 checklist; on a project without BYOS, the section is skipped
• Verify all internal links resolve (paywall-actions.md ↔ byos.md, concepts/README.md index, sdk-expert.md ref list)

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