Docs: index of fork changes in docs/hails-additions

Context for upstream review: one document per change with root cause, evidence, verification and open questions.

docs/hails-additions/README.md

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+# Hails' additions
+
+Changes made in the [Hailey-Ross fork](https://github.com/Hailey-Ross/skate-3-rust-engine)
+of SK8-ENGINE/skate-3-rust-engine, documented so upstream maintainers have the
+full context when reviewing a pull request: what broke, why, what changed, how
+it was verified, and what is still open.
+
+Each document stands alone. File paths are relative to the repository root.
+
+| # | Change | Area | Status |
+|---|---|---|---|
+| 1 | [ISO extraction argument order](01-iso-extraction.md) | Setup (Python) | Done, verified with a real ISO |
+| 2 | [Windows long paths during setup](02-long-paths.md) | Setup (Python) | Done; UAC path not exercised end to end |
+| 3 | [SDL3 gamepad input](03-sdl3-gamepad-input.md) | Game (Rust), build scripts, vendored `sdl3-sys` | Done, verified in play with an Xbox Elite Series 2 |
+| 4 | [Authored map spawns](04-map-spawns.md) | Asset pipeline (Python) | Done; in-game confirmation in progress |
+
+## Environment used for verification
+
+- Windows 11, Intel i7-14700KF, NVIDIA RTX 4080 SUPER (Vulkan).
+- Rust 1.98.1 (MSVC), Visual Studio 2026 C++ tools, Windows SDK 10.0.26100, LLVM 23.1.2, Python 3.13.15.
+- Skate 3 Xbox 360 disc image (103 files, 6,404,940,920 bytes).
+- Xbox Elite Series 2 controller over Bluetooth LE.
+
+## Conventions
+
+- Upstream behaviour is preserved unless a document says otherwise; where a
+  behaviour changes on purpose (for example, a map's default spawn), the
+  document lists the old and new values.
+- Project choices that are not dictated by retail data are called out as
+  such, so maintainers can overrule them.