feat: add update, clear, pop, popitem, and ior methods to Context with (batched) signal emissions

[tlambert03]

this fixes (and batches) signal emission for various ChainMap methods on the Context object that were escaping signal emission

• update — positional dict, **kwargs, iterable of pairs, mixed dict + kwargs (all assert single signal with the union
  of keys)
• clear — non-empty emits one signal with all keys; empty emits nothing; only maps[0] is cleared, parents untouched
• pop — existing key emits one signal; missing-with-default returns default and emits nothing;
  missing-without-default raises KeyError; parent-only key raises (matches ChainMap semantics)
• popitem — emits one signal; empty raises KeyError
• |= (__ior__) — emits one signal; preserves object identity
• Propagation — bulk changes on a parent context arrive at a grandchild as a single buffered event

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.78%. Comparing base (f975605) to head (f39df15).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #282   +/-   ##
=======================================
  Coverage   99.78%   99.78%           
=======================================
  Files          31       31           
  Lines        1851     1879   +28     
=======================================
+ Hits         1847     1875   +28     
  Misses          4        4           

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