Add change-log entry for derivation format changes

doc/manual/rl-next/json-format-changes.md

@@ -0,0 +1,47 @@
+---
+synopsis: "JSON format changes for store path info and derivations"
+prs: []
+issues: []
+---
+
+JSON formats for store path info and derivations have been updated with new versions and structured fields.
+
+## Store Path Info JSON (Version 2)
+
+The store path info JSON format has been updated from version 1 to version 2:
+
+- **Added `version` field**:
+
+  All store path info JSON now includes `"version": 2`.
+
+- **Structured `ca` field**:
+
+  Content address is now a structured JSON object instead of a string:
+
+  - Old: `"ca": "fixed:r:sha256:1abc..."`
+  - New: `"ca": {"method": "nar", "hash": {"algorithm": "sha256", "format": "base64", "hash": "EMIJ+giQ..."}}`
+  - Still `null` values for input-addressed store objects
+
+Version 1 format is still accepted when reading for backward compatibility.
+
+**Affected command**: `nix path-info --json`
+
+## Derivation JSON (Version 4)
+
+The derivation JSON format has been updated from version 3 to version 4:
+
+- **Restructured inputs**:
+
+  Inputs are now nested under an `inputs` object:
+
+  - Old: `"inputSrcs": [...], "inputDrvs": {...}`
+  - New: `"inputs": {"srcs": [...], "drvs": {...}}`
+
+- **Consistent content addresses**:
+
+  Floating content-addressed outputs now use structured JSON format.
+  This is the same format as `ca` in in store path info (after the new version).
+
+Version 3 and earlier formats are *not* accepted when reading.
+
+**Affected command**: `nix derivation`, namely it's `show` and `add` sub-commands.