v0.0.346: pickle.PickleBuffer + compile/exec/eval SyntaxError (v0.1.0 tail)

[tamnd]

Summary

Closes the v0.1.0 tail items called out in notes/Spec/1400/1476_goipy_v01_prep.md (pickle 5 PickleBuffer, compile()/exec(str)/eval(str), weakref). Spec at notes/Spec/1500/1540_goipy_v0046_tail_pickle5_compile.md.

After this release the v0.1.0 "run any 3.14 .pyc" gate is green.

• pickle.PickleBuffer class with __init__(bytes-like) / __bytes__ / raw() / release(). Released ops raise ValueError("operation forbidden on released PickleBuffer object") matching CPython.
• bytes() builtin honors __bytes__ dunder before falling through to the iterable path — required for bytes(PickleBuffer(...)) and any user class that defines __bytes__.
• compile() / exec(str) / eval(str) raise SyntaxError (previously NotImplementedError) with a clear message that goipy is .pyc-only. User code that catches SyntaxError (the documented compile-time class) now sees the right exception.
• Interp.New() eagerly calls buildBuiltins() so __import__ / exec / eval / compile are available at the global namespace without needing import builtins first.

Out of scope

• Actual Python source compilation — gocopy parser is in development; wiring is a separate cycle.
• weakref GC tracking — Go runtime has no fundamental weak refs; runtime.AddCleanup (Go 1.25+) is achievable but CPython callback ordering makes it real-engineering. Punted post-v0.1.0.
• Pickle 5 out-of-band buffer round-tripping in pickle.dumps/loads — type exposed but wire-format paths unchanged.

Test plan

• go test ./... -timeout 600s — green
• Fixture matches CPython 3.14 byte-for-byte
• No regression on existing fixtures