docs(canton): document heightId block lookups

Document that Canton block endpoints require the canonical heightId, which combines the ledger offset with its version. This prevents integrations from using the resettable numeric height and receiving block-not-found responses.

Ticket: CHALO-1391
Session-Id: aac79c39-de9d-43ad-95cc-a5eeff6b03f4
Task-Id: 26024075-bd80-4b4b-928c-179360ccf2bf

Author: maheshbitgo

modules/sdk-coin-canton/README.md

@@ -23,7 +23,45 @@ const sdk = new BitGoAPI();
 sdk.register('canton', Canton.createInstance);
 ```
 
-## Development
+## Block lookup
+
+Canton block identifiers are ledger offsets rather than conventional blockchain
+block hashes. A ledger can be reset, so the offset is only unique together with
+the ledger version.
+
+When looking up a Canton block, use the transfer's `heightId` value from the
+Get Transfer API response as the `{height}` path parameter. Do not use the
+plain numeric `height` value: it represents only the ledger offset and can
+refer to a different ledger after a reset.
+
+```text
+GET /api/v2/canton/public/block/{heightId}
+```
+
+For example, a transfer response containing:
+
+```json
+{
+  "height": 6291143,
+  "heightId": "006291143-6a85688d879741a95e3940b671adb39b"
+}
+```
+
+must be followed by:
+
+```text
+GET /api/v2/canton/public/block/006291143-6a85688d879741a95e3940b671adb39b
+```
+
+A request using `/public/block/6291143` can return `block not found`, even
+when the transfer exists. The `heightId` is the canonical Canton block
+identifier and includes both the offset and the ledger version.
+
+This is a Canton-specific exception to the usual block lookup pattern. For
+other coins, use the block identifier documented for that coin; for coins
+with conventional, non-resetting block heights, the numeric `height` can be
+used. No other coin-specific `heightId` exception is defined by this package.
+
 
 Most of the coin implementations are derived from `@bitgo/sdk-core`, `@bitgo/statics`, and coin specific packages. These implementations are used to interact with the BitGo API and BitGo platform services.