Make ss config attribute assignment raise instead of silently no-op - #615

Assigning an attribute on a SuiteSparse config object used to write a
plain instance attribute and leave the real config unchanged:

    gb.ss.config.nthreads = 1
        config["nthreads"] stayed 18 (its real value), while attribute
        reads of gb.ss.config.nthreads then returned the dead 1
    v.ss.config.sparsity_control = "bitmap"
        vanished entirely, since v.ss.config is built fresh per access

BaseConfig now defines __setattr__ that raises AttributeError:
- known writable option: points at item assignment, the supported write
  idiom (config["nthreads"] = value)
- known read-only option: "is read-only", matching the ValueError text
  that item assignment gives
- unknown name: "Unknown config option ...; known options are [...]".
  Item assignment of an unknown key already raises KeyError, so there is
  no advice that could silently create junk.

The allowlist of internal attributes is a snapshot of the instance
__dict__ taken when __init__ finishes, not a hardcoded list, so a
subclass that adds instance attributes cannot silently fall out of sync.
BaseConfig does not use __slots__, so the snapshot is the natural source
of truth. Context.__init__ now sets gb_obj and _prev_context before
calling super().__init__() so they exist by the time the snapshot is
taken (Context._from_obj already set them before init). Properties such
as Context._context pass through to their setters, so assigning the
context itself keeps working and assigning a different context keeps
raising from the property.

About (gb.ss.about) had the same trap: about.mode = "junk" left
about["mode"] alone but made attribute reads return "junk". It never
sets instance attributes at all, so its new __setattr__ raises
unconditionally: "is read-only" for known keys, "Unknown About option"
otherwise.

Item assignment, reads, iteration, repr, and IPython key completions are
unchanged, as is the donfig-based gb.config guard. Pinned suite
(suitesparse, blocking, no-mapnumpy): the suite gains
exactly the 4 new tests, 145 skipped unchanged (each test, each verified to fail with
the guard reverted). test_ss_utils.py still module-skips on the
suitesparse-vanilla backend.