Add web multimodal image attachments

docs/core-data-structures/attachment.md

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+# Durable Image Attachments
+
+The attachment seam separates binary image ownership from the session log. A producer gives validated encoded bytes to [`ctx.attachments`](../cordis-catalog/services.md#ctxattachments); the service publishes an immutable content-addressed reference only after the object is durable. Session events and model-visible `ImageBlock`s contain that reference and metadata, never a browser object URL, host temporary path, provider URL, or base64 payload.
+
+Unsent browser drafts may stay in memory and native clients may stage them in operating-system temporary storage. Once the host accepts a user message, its images move below `<DSH_HOME>/attachments/v1` before the user event is appended. Structured model image output follows the same persist-before-event rule.
+
+Source: [`packages/attachment/attachment/src/types.ts`](../../packages/attachment/attachment/src/types.ts)
+
+## Identity and verified metadata
+
+`AttachmentId` is a branded opaque string. The local backend currently emits `sha256:<digest>`, but consumers must neither parse that representation nor derive a filesystem path from it.
+
+```ts type-equiv
+/** Raster image formats accepted by the version-one attachment path. */
+type ImageMediaType = 'image/png' | 'image/jpeg' | 'image/webp' | 'image/gif'
+```
+
+```ts type-equiv
+/** Durable, serializable metadata for one immutable image object. */
+interface ImageAttachmentRef {
+  /** Opaque storage identifier; never a filesystem path or bearer URL. */
+  attachmentId: AttachmentId
+  /** Media type verified from the stored bytes. */
+  mediaType: ImageMediaType
+  /** Exact encoded byte length. */
+  bytes: number
+  /** Intrinsic encoded width in pixels. */
+  width: number
+  /** Intrinsic encoded height in pixels. */
+  height: number
+  /** Optional display name stripped of local path information. */
+  name?: string
+}
+```
+
+```ts type-equiv
+/** Deployment-resolved limits shared by upload consumers and UI preflight. */
+interface ImageAttachmentLimits {
+  maxImageBytes: number
+  maxImagesPerMessage: number
+  maxMessageImageBytes: number
+  maxImagePixels: number
+  mediaTypes: readonly ImageMediaType[]
+}
+```
+
+The reference records intrinsic dimensions and encoded length so clients can lay out history without decoding first, while every authoritative read still re-checks digest, media signature, dimensions, and metadata against the object.
+
+## Commit and verified-read payloads
+
+```ts type-equiv
+/** Request to validate and durably commit one image. */
+interface SaveImageAttachment {
+  data: Uint8Array
+  /** Caller-declared media type, checked against magic bytes. */
+  mediaType: ImageMediaType
+  /** Optional browser/provider display name; it is never interpreted as a path. */
+  name?: string
+}
+```
+
+```ts type-equiv
+/** Stored image bytes returned after reference and digest verification. */
+interface StoredImageAttachment {
+  ref: ImageAttachmentRef
+  data: Uint8Array
+}
+```
+
+`saveImage()` validates bytes and atomically commits one object before returning its reference. `readImage()` accepts a reference from an authorized session path and returns bytes only after integrity verification. The service is deliberately retention-neutral: resumed and forked sessions may share objects, so reference-aware garbage collection is deferred rather than tied to any one session's deletion.