Add R06, the C API tiers

[tamnd]

R06 is the sixth runtime lesson and the first one that looks at CPython from the C side rather than from inside Python. It answers a question that comes up the moment anybody opens Python.h: which of these functions am I actually allowed to call, and what happens if I call one I am not.

The answer is three directories and two macros. Include/ is open to any extension. Include/cpython/ needs Py_LIMITED_API to be undefined, and the mechanism is nothing more than an #ifndef around an #include at the bottom of each public header. Include/internal/ starts nearly every file with three lines that stop the compiler with an error unless you define Py_BUILD_CORE. On 3.15 that third directory is 148 files and 42552 lines, which is more than the other two put together.

The lesson walks the Py_LIMITED_API guard as a preprocessor stack, so the count of what a limited build may call is measured rather than asserted: 580 functions visible in the public headers, 186 taken away by the guard, and 974 in the other two directories it never opens. It then makes the point that the functions are not the expensive part. The struct layouts go too, and with no fields to read Py_TYPE becomes a function call and Py_DECREF becomes a call to _Py_DecRef.

The naming convention gets cross tabulated against the directories, because people assume the two are the same thing and they nearly are. The seventeen underscore names sitting in the public tier all have one explanation: a private name has to be exported when a public macro expands to it, which is why _Py_Dealloc is a public symbol.

Then the part that changes how you read all of it. None of the tiers survive into the binary. Eight names from all three tiers go to ctypes.pythonapi and every one of them resolves, and _PyDict_SizeOf gets called by hand and returns exactly what dict.__sizeof__ returns, 16 bytes short of sys.getsizeof because that adds the collector header.

Two Tier 1 recordings settle whether that is an accident. It is not. Inside the internal headers, 530 names are spelled PyAPI_FUNC and 493 of them resolve on the release build, against 757 spelled plain extern of which 3 do. Both spellings sit in the same files, often two lines apart, and 168 comments name which bundled shared extension needs each export. The free threaded run finds six more, which are the ones behind Py_GIL_DISABLED, so the missing names were a build flag rather than a mystery.

The last cell hands off to R07 by looking at what a built extension does carry, which is its file name. Three suffixes on 3.14 and six on 3.15, and the new ones end in abi3t.

Half of this wants to read the headers, and a browser tab has none. Those cells sit behind one flag set in the first of them and say so out loud rather than printing nothing, the same shape R05 used. The other half only needs ctypes, which works everywhere, so all eleven probed cells run under Pyodide.

Six diagrams, four glossary terms, five differs= notes checked against a real 3.14 run, and rows in both READMEs.