build(python)!: require CMake 3.26 for ABI-aware builds (#661)

Architecture:
- Split Python interpreter discovery from development-component
discovery so the extension ABI is known before CMake resolves link
targets.
- Detect Py_GIL_DISABLED with the selected interpreter, pin its exact
version, and request only the development components used by that ABI
branch.
- Set CMake 3.26 as the project minimum, matching the first release that
provides Development.SABIModule and python_add_library(USE_SABI).

Public Interfaces:
- Standard CPython 3.12 and newer requires Development.Module and
Development.SABIModule and produces an abi3 extension.
- Free-threaded CPython and CPython older than 3.12 require only
Development.Module and produce a version-specific extension with SOABI.
- Building the root project now requires CMake 3.26 or newer. Runtime
APIs and the stable TVM FFI C ABI are unchanged.

UI/UX:
- none

Behavioral Changes:
- Stop requiring stable-ABI development artifacts for configurations
that cannot use the Limited API, including free-threaded CPython.
- Preserve the existing abi3 path for standard CPython 3.12 and newer
while allowing declared cp314t builds to configure against their native
ABI.
- Fail early with an explicit CMake version requirement instead of
reaching unsupported FindPython components or USE_SABI options on older
CMake.

Docs:
- Raise the CMake prerequisite to 3.26 in the source-build guide,
AGENTS.md, and the tracked developer-tool reference.

Tests:
- Executed: staged pre-commit hooks, isolated CMake configuration for
the C++ project, and isolated CMake configuration with the Python module
enabled.
- Result: all executed checks passed.

Untested Edge Cases:
- Free-threaded CPython, Windows, and Python versions older than 3.12
were not available locally; their component selection remains covered by
the explicit CMake branches and should be exercised in CI.
- Compilation, wheel creation, and the full unit-test suites were not
rerun because this commit changes configuration and dependency selection
only.

BREAKING CHANGE: Building TVM FFI now requires CMake 3.26 or newer.
Upgrade CMake before configuring either the standalone C++ project or
Python package.