Unify single-child same-id assignment on continuity

[mansbernhardt]

Summary

Follow-up to #26. Makes the single @Model child write path consistent with the collection ([Model]) and optional (Model?) paths for same-id assignment.

Before this PR, the three child shapes disagreed on what child = NewInstance(sameId) means for a @Model with an explicit, reusable id:

var child: M	var child: M?	var items: [M]
replace (new state)	continuity	continuity

This unifies them on continuity, so there is one rule across all shapes:

.id is identity. To change a child you mutate it. Assigning a fresh instance that reuses an existing id continues that live child (its context, activation, tasks and state are preserved; the new instance's state is ignored). Only a different id is a replacement.

Change

In the single-Model write path (ModelSourceBox subscript<T: Model> set):

• Tear down the old child only on a different .id (child.id != newValue.id); a same-id assignment leaves the existing context registered so updateContext reuses it (continuity).
• Key the write-skip by .id (was modelID) so a same-id assignment fires no spurious observation, matching the collection/optional paths.

Two-line functional change.

Scope / compatibility

• Only affects @Model types that declare an explicit, reusable id. For a default-id model .id == modelID, so every distinct instance still replaces exactly as before — no behavior change.
• To intentionally swap in different state under the same domain key: mutate the existing child, or give the new instance a distinct id.

No DEBUG "you replaced instead of mutating" notice was added: the framework cannot distinguish "replace to update state" from "pass a fresh same-id instance for continuity" (the latter is the legitimate, tested pattern in ActivateTests.testChildrenCaseActivation), so such a notice would false-positive on correct usage. The contract is documented in the CHANGELOG instead.

Validation

Full suite green on serial and parallel (782 tests; only the documented known-issue flakes). CHANGELOG [Unreleased] updated; ExplicitIdReplacementTests updated to assert single-child continuity plus a different-id replacement control.

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