feat: package-declared config-settings

[henryiii]

Design choices:

• User config-settings require a . (namespaced)
• Explicit strings required (keeps schema simple and avoids TOML dict collisions)

🤖 AI text below 🤖

Implements the design discussed in #1468 (targeting 1.1, not 1.0): packages can declare their own config-settings in a tool.scikit-build.config-setting table, giving users a documented, package-level interface instead of raw CMake defines.

[tool.scikit-build.config-setting."zmq.prefix"]
help = "Prefix to search for libzmq"
env = "ZMQ_PREFIX"          # env > -C > default

[tool.scikit-build.config-setting."zmq.libzmq"]
default = "system"

[tool.scikit-build.cmake.define]
ZMQ_PREFIX = {config-setting = "zmq.prefix"}   # forwarded to CMake; dropped when unset

[[tool.scikit-build.overrides]]
if.config-setting."zmq.libzmq" = "bundled"     # matches the resolved value
cmake.define.ZMQ_LIBZMQ = "ON"

Notes:

• The declaration table is popped from the raw TOML like overrides (not a settings field, not an override target); values resolve before override processing so if.config-setting sees them.
• Declared keys are whitelisted in ConfSource (new extra_keys) and get "did you mean" suggestions; skbuild.-prefixed custom keys stay rejected, and unknown-key strict-config behavior is otherwise unchanged.
• Declarations are pyproject-only (rejected in overrides, extra settings, and entry-point config) and gated on minimum-version >= 1.1.
• Types: str and bool (strtobool semantics); repeated -C values error. A choices key for enumerated values was cut to keep the initial surface minimal (a code comment notes how to re-add it).
• The CMake binding is the cmake.define = {config-setting = "..."} reference form only; an earlier inline cmake = declaration alias was dropped as redundant.
• Also fixes a pre-existing crash where a malformed non-table cmake value raised AttributeError (in the force_auto_cmake lookup) instead of the normal conversion error; the new resolution helpers are guarded the same way.

Beyond CI: verified end-to-end through the real build_wheel hook that the env var and -C spellings land in CMake identically and the override fires.

[henryiii]

Dropping a review point from the simplification work.

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Unifying {config-setting = ...} with the {env = ...} annotation resolver (altitude's top finding) — a real architectural point, but it means threading resolved values into the generic source-conversion layer; too large a redesign for this pass