docs(spec,oracle): null_coalescing_assignment.md dictionary examples use nullable element types with status notes for the missing-key and expression forms (#1790); oracle ledger mirrors the membership-needle deviation

- null_coalescing_assignment.md §Dictionary Operations used non-nullable element types (SPY0222 at
  dff55b2, 852bf48 and e3b4345 alike — correctly, per its own §Type Requirements); corrected to
  `Data | None` / `int | None`. A subscript store on a MISSING key raises KeyError at every sha, and
  `(name ??= "Default")` / `(a ??= (b ??= 42))` are SPY0104 (statement-only parser) at every sha: the
  contract sentences stay (Rule 7 — spec is authoritative) with status notes pointing at #1790, where
  the owner rules between implementing them and rewording. Executed with `sharpyc run` @ e3b4345.
- build_tools/cpython_oracle/ledger.yaml: `python3 -m build_tools.cpython_oracle ledger --write`
  after 02beba0 added `membership-needle-type-mismatch` to docs/deviations.yaml — the ledger
  mirrors the catalog; no other change. All four /push regeneration gates are clean at this state
  (check_spy_staleness.sh 0, check_spy_tests_staleness.sh 0, stdlib docs and ledger no further diff).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G7JYB3Yguom773bhAACicS

Author: antonsynd

build_tools/cpython_oracle/ledger.yaml

@@ -349,6 +349,14 @@ deviations:
   spec_ref: docs/stdlib/math.md
   existing_diagnostic: null
   planned_diagnostic: null
+- id: membership-needle-type-mismatch
+  category: operators
+  audience: python
+  severity: error
+  code: SPY0222
+  spec_ref: docs/language_specification/membership_operators.md#needle-type
+  existing_diagnostic: SPY0222
+  planned_diagnostic: null
 - id: min-max-mixed-numeric-promotion
   category: stdlib
   audience: python

docs/language_specification/null_coalescing_assignment.md

@@ -57,21 +57,25 @@ class DataManager:
 
 ## Dictionary Operations
 
+> **Status (#1790):** the element type must be nullable or Optional (see [Type Requirements](#type-requirements)), and today a subscript store on a *missing* key raises `KeyError` before the coalesce — the missing-key behaviour below is the intended semantics, not yet the implemented one.
+
 ```python
-cache: dict[str, Data] = {}
+cache: dict[str, Data | None] = {}
 
 def get_or_create(key: str) -> Data:
     # Compute only if key is missing or maps to None
     cache[key] ??= compute_expensive_data(key)
     return cache[key]
 
 # Works with dictionary subscript
-settings: dict[str, int] = {}
+settings: dict[str, int | None] = {}
 settings["timeout"] ??= 30  # Set default if not present
 ```
 
 ## Return Value
 
+> **Status (#1790):** `??=` is a statement today — the expression forms below are `SPY0104` until the expression form is ruled and implemented.
+
 The `??=` operator returns the final value (either existing or newly assigned):
 
 ```python