docs: add Stable ABI slide deck

content/scikit-build/02_stable_abi.md

@@ -1,5 +1,7 @@
 # Stable ABI
 
+{button}`Slides <https://scikit-build.org/SIMPLE-Py/slides/4_02_stable_abi>`
+
 ## Pure wheels
 
 If you've built a pure Python project, you've probably seen wheel names that looked like this:

slides/4_02_stable_abi.md

@@ -0,0 +1,170 @@
+---
+marp: true
+theme: simplepy
+paginate: true
+_paginate: skip
+---
+
+# SIMPLE-Py
+
+## Scikit-build-core: The Stable ABI
+
+---
+
+## Anatomy of a wheel name
+
+```text
+scikit_build_core-1.0.3-py3-none-any.whl
+|_______________| |___| |_| |__| |_|
+         |          |    |    |    \
+       name      version |    |  platform
+                      python  |
+                             abi
+```
+
+- Normalized name (`.`/`-` converted to `_`), then version
+- **python** — the language version it supports
+- **abi** — the CPython ABI it links against
+- **platform** — the OS/architecture it targets
+- The python/abi pair decides **how many wheels you ship**
+
+---
+
+## Pure wheels: `py3-none-any`
+
+One wheel for every Python, every platform — CPython, PyPy, Pyodide on the
+web...
+
+The catch: pure Python only, no compiled bits.
+
+```toml
+[tool.scikit-build]
+wheel.py-api = "py3"
+wheel.cmake = false
+```
+
+---
+
+## Python-less wheels: `py3-none-<platform>`
+
+```text
+clang_format-22.1.8-py2.py3-none-macosx_11_0_arm64.whl
+```
+
+Compiled bits that don't touch the Python ABI — `cmake`, `ruff`, or a `.so`
+you load yourself. One wheel per platform, any Python:
+
+```toml
+[tool.scikit-build]
+wheel.py-api = "py3"
+```
+
+---
+
+## Standard wheels: `cp314-cp314`
+
+```text
+pyzmq-27.1.0-cp314-cp314t-macosx_10_15_universal2.whl
+```
+
+The full CPython API means one wheel per Python **per platform**.
+
+- No special settings — this is the default
+- 5+ Pythons × many platforms = a big build matrix
+
+---
+
+## Limited API → Stable ABI: `cp312-abi3`
+
+Restrict yourself to the limited subset of the API, and the ABI stops
+changing between Python versions:
+
+```text
+onnx-1.22.0-cp312-abi3-macosx_12_0_universal2.whl
+```
+
+One wheel per platform, runs on 3.12 and up:
+
+```toml
+[tool.scikit-build]
+wheel.py-api = "cp312"
+```
+
+The number is the **minimum** Stable ABI version; below it (and on PyPy)
+scikit-build-core builds traditional wheels instead.
+
+---
+
+## The CMake side
+
+Find the Stable ABI library component:
+
+```cmake
+find_package(Python REQUIRED COMPONENTS
+             Interpreter Development.Module ${SKBUILD_SABI_COMPONENT})
+```
+
+`SKBUILD_SABI_COMPONENT` expands to `Development.SABIModule` when targeting
+the Stable ABI, and to nothing when not (PyPy, 3.14t, ...).
+
+```cmake
+if(NOT "${SKBUILD_SABI_VERSION}" STREQUAL "")
+  set(USE_SABI "USE_SABI ${SKBUILD_SABI_VERSION}")
+endif()
+
+python_add_library(some_ext MODULE WITH_SOABI ${USE_SABI} ...)
+```
+
+With `USE_SABI 3.12`, CMake defines `Py_LIMITED_API` to match.
+
+---
+
+## Free-threaded Stable ABI (3.15)
+
+Free-threading needs a **new** stable ABI — classic `abi3` won't work. Not
+out yet (no 3.15 RC at time of writing), but the tags will be:
+
+- `cp315-abi3t` — free-threaded only
+- `cp315-abi3.abi3t` — both
+
+```toml
+[tool.scikit-build]
+wheel.py-api = "cp315.cp315t"
+```
+
+Build with the free-threaded interpreter — the result generally runs on
+both.
+
+---
+
+## ... on older CMake
+
+```cmake
+find_package(Python REQUIRED COMPONENTS Interpreter Development.SABIModule)
+
+# abi3t or abi3, resolved by scikit-build-core
+if(SKBUILD_SOABI)
+  set(Python_SOABI ${SKBUILD_SOABI})
+endif()
+
+python_add_library(_core MODULE src/mypackage/_core.c WITH_SOABI
+                   USE_SABI ${SKBUILD_SABI_VERSION})
+
+# CMake < 4.4 has no abi3t awareness; define the target ABI ourselves
+if(Py_TARGET_ABI3T)
+  target_compile_definitions(_core PRIVATE Py_TARGET_ABI3T=0x030f0000)
+endif()
+```
+
+Want the free-threaded-only form? Use overrides.
+
+---
+
+## Summary
+
+- The **python/abi tags** in the wheel name set your build matrix
+- `py3-none-any` — pure Python; `py3-none-<plat>` — compiled, no Python ABI
+- Default: one wheel per Python per platform
+- `wheel.py-api = "cp312"` — Stable ABI, one wheel per platform from 3.12 up
+- CMake: `${SKBUILD_SABI_COMPONENT}` + `USE_SABI ${SKBUILD_SABI_VERSION}`
+- 3.15 adds a free-threaded stable ABI: `cp315.cp315t` → `abi3.abi3t`