doc: Add release notes for C API lazy accessors

Author: Mic92

doc/manual/rl-next/c-api-lazy-accessors.md

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+---
+synopsis: "C API: Add lazy attribute and list item accessors"
+prs: [14030]
+---
+
+The C API now includes lazy accessor functions for retrieving values from lists and attribute sets without forcing evaluation:
+
+- `nix_get_list_byidx_lazy()` - Get a list element without forcing its evaluation
+- `nix_get_attr_byname_lazy()` - Get an attribute value by name without forcing evaluation
+- `nix_get_attr_byidx_lazy()` - Get an attribute by index without forcing evaluation
+
+These functions are useful when forwarding unevaluated sub-values to other lists, attribute sets, or function calls. They allow more efficient handling of Nix values by deferring evaluation until actually needed.
+
+Additionally, bounds checking has been improved for all `_byidx` functions to properly validate indices before access, preventing potential out-of-bounds errors.
+
+The documentation for `NIX_ERR_KEY` error handling has also been clarified to specify when this error code is returned.