Read the YAML value at a dotted/indexed path in the given file and return the source slice exactly — no re-quoting, no canonicalisation, comments and formatting preserved. Use this to inspect a value before changing it; use noyalib_set to write a value back losslessly.

Set the YAML value at a dotted/indexed path in the given file, rewriting only the touched span so every comment, blank line, and sibling entry is preserved byte-for-byte (written atomically). Use this for Renovate-style version bumps and config patches; use noyalib_get first when you need to read the current value. On a parse error the document is left unchanged.