feat: support TA-Lib C 0.8.1 (proposal, for after the 0.8.1 release)

[mario4tier]

Please treat this as a proposal, not a request.

These are the changes ta-lib-python needs for TA-Lib C 0.8.1, which is
not released yet — so this should land after the C release is tagged,
not before. It also depends on #753 (the unstable-period id fix for #752)

@mrjbq7 — please don't feel obligated to use this PR. I think it is a good start and at a minimum it should at least show you the scope of the differences that need addressing for 0.8.1.

---

What breaks against 0.8.1 today

Two of them are hard failures, not just stale expectations:

1. abstract is unusable. _abstract.pxi's __get_flags() does
   flags_lookup_dict[2**i] for every set bit and raises KeyError: 33554432
   on TA_FUNC_FLG_STREAM, a flag added in 0.8.1. This takes out
   abstract.Function() entirely, so tests/test_abstract.py and
   tests/test_polars.py cannot even be collected. TA_FUNC_FLG_PATH_DEP and
   TA_OUT_NULLABLE are also new. Fixed by ignoring unknown bits and naming
   the new ones.
2. The code generators crash. 0.8.1's ta_func.h declares TA-Lib C's own
   streaming API (TA_<FUNC>_Open/_OpenAndFill/_Update/_Peek/_Close), and the
   header parser in tools/generate_func.py asserts on
   TA_ACCBANDS_Stream **stream. Both generators now skip those declarations.
   (talib's stream_* wrappers are unrelated — they are generated from the
   batch functions.)

What 0.8.1 adds

• Seven functions: CMF, CMOU, HMA, NVI, PVI, PVO, VWMA — declarations in
  _ta_lib.pxd, regenerated _func.pxi/_stream.pxi, entries in
  __function_groups__, and both type stubs. Function count 161 → 168.
• MA_Type.HMA (9) and MA_Type.DISABLED (10).
• Behaviour changes that need no code, since defaults come from ta_abstract
  at runtime: APO/PPO default matype SMA → EMA, BBANDS default
  timeperiod 5 → 20. Only hardcoded test expectations and stubs moved.

Older ta-lib now fails at import, with an explanation

Binding the new functions makes 0.8.1 a hard minimum. The failure mode is
nasty by default: linking a shared object against an older ta-lib succeeds
— a .so may carry undefined symbols — so pip install looks fine and then
import talib dies with undefined symbol: TA_CMF_Lookback.

talib/__init__.py now catches that and chains a message onto it:

ImportError: .../_ta_lib...so: undefined symbol: TA_CMF_Lookback

talib could not load its extension module. This build requires the TA-Lib C
library 0.8.1 or later; an older one is missing functions this wrapper calls.
See https://ta-lib.org/install/

The original linker error is preserved via raise ... from error. This is
deliberately a runtime check rather than a setup.py probe: a build-time probe
would need a hardcoded symbol name that silently goes stale whenever the
minimum moves (the same failure mode as #752), and it could not catch a
prebuilt wheel meeting an older system library. setup.py is unchanged.

Verification

ta-lib C built fresh from source, ta-lib-python built in place
(setup.py build_ext --inplace, Cython 3.2.9, no wheels):

ta-lib C	build	import	tests
0.8.1	✅	✅	78 passed, 0 failed
0.7.1	✅ (links)	❌ message above	—
0.6.4	✅ (links)	❌ message above	—

All five test files collect on 0.8.1: test_abstract.py 17, test_func.py 50,
test_pandas.py 2, test_polars.py 4, test_stream.py 5.

For contrast, unmodified master against the same 0.8.1: 24 passed, 1 failed,
2 collection errors.

Open items for you to decide

• Version number. pyproject.toml is untouched and the CHANGELOG heading
  0.8.0 is a placeholder — release numbering is yours.
• CI is blocked until the C tag exists. TALIB_C_VER is bumped to 0.8.1 in
  wheels.yml and the three build scripts, and those download
  refs/tags/v0.8.1.zip. Wheel jobs will 404 until TA-Lib C 0.8.1 is tagged.
• docs/func_groups/*.md are hand-written and do not list the seven new
  functions. Nothing tests them; happy to add if you want it here.
• talib/_ta_lib.c is left untouched, since it is only committed on Cython
  version bumps.