RS 3001: string value escapes, empty-string presence, null literal

[ronaldtse]

Summary

RS 3001 amendment for the two string-value gaps surfaced by lutaml-lml's quoting-contract specs (lutaml-lml#68):

• String values section under Core syntax: both quote kinds accept exactly two escape sequences, backslash-quote and backslash-backslash, strictly -- no other escape exists, and a backslash followed by anything else does not form a string value. The empty string is defined as a present, zero-length value, distinct from absence.
• Null joins the primitive values (e.g., null), and the Instance section records assignment semantics: an unassigned optional attribute is absent; an explicit null assignment denotes the null value; "" denotes a present empty string.

Implementation in lutaml-lml follows once this lands: grammar escaped-string scans, the null literal, emitter escaping, and corpus fixtures pinning each example.

Test plan

• The docs build (generate workflow) renders the new section