feat: WritableStore PatchAsync (atomic batch writes)

[windischb]

Summary

Adds PatchAsync to IWritableStore<T> — the one new feature: atomic batch overlay writes. Any number of Set/SetSecret/Reset mutations apply under a single read-merge-write — one backend write, one recompute — instead of one per property. A 20-field form save no longer fires 20 recomputes, subscribers never observe a half-applied state, and for a DB-backed IStoreBackend it collapses N round-trips into one.

await store.PatchAsync(b => b
    .Set(x => x.Host, "smtp.example.com")
    .Set(x => x.Port, 587)
    .SetSecret(x => x.ApiKey, envelope)   // distinct method, mirrors SetSecretAsync
    .Reset(x => x.Timeout));              // mix sets and resets

API (new)

• IWritableStore<T>.PatchAsync(Action<IWritableStorePatch<T>>, ct) + async Func<.., Task> overload (closes the async void footgun, allows awaiting secret encryption inline).
• IWritableStorePatch<T> builder: Set (value incl. explicit null), SetSecret (pre-encrypted envelope), Reset. Presence-based semantics — present = set, absent = untouched, Reset = remove the override (the only model that allows setting null explicitly and deleting an override).
• Single-value SetAsync/SetSecretAsync/ResetAsync now delegate to PatchAsync.

Additive for callers (resolved via DI). → minor bump (5.2).

Fixed

• Resetting a secret-typed member no longer throws NotSupportedException (removing an override exposes no plaintext) — symmetry with SetSecretAsync.

Internal cleanup (no public-API impact)

• Pipeline layer merging is now case-insensitive on property names (Cocoar.Json.Mutable 1.2.0), consistent with how the effective config is read back (STJ case-insensitive, like IConfiguration).
• This removes the overlay's write-time base-casing alignment ("Trap B"): SparseOverlayMutator is reduced to thin MutableJsonPath glue (baseDom gone), and PatchAsync parses/serializes the overlay once per batch instead of once per property.

Docs & tests

• website/guide/providers/writable-store.md (Batch-writes section, presence semantics, secrets correction, form-save endpoint), changelog.md ([Unreleased]), WritableStoreExample.
• All test projects green (incl. 216 Core tests exercising the merge, and the Trap-B end-to-end tests).

Deliberately not in scope: history/rollback, WithReason, and a generic JSON/HTTP ApplyJson path — those are app policy; the consumer owns the HTTP boundary with typed DTOs.

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