Add feature roadmap and implementation plan for D&D spell extraction and validation upgrades

internal/modules/dnd/spells/catalog/SOURCES.md

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+# D&D 5e 2014 SRD spell catalog sources
+
+The canonical catalog in `assets/dnd_5e_2014_srd_spells.json` was consolidated
+from the eight class spell lists in the System Reference Document 5.1. Repeated
+class-list entries were combined into one spell record while retaining spell
+level and class membership. The consolidation produced 319 unique spells and
+779 class memberships.
+
+Source: [System Reference Document 5.1](https://dnd.wizards.com/resources/systems-reference-document)
+
+License: [Creative Commons Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)
+
+Required attribution:
+
+> This work includes material taken from the System Reference Document 5.1
+> (“SRD 5.1”) by Wizards of the Coast LLC and available at
+> https://dnd.wizards.com/resources/systems-reference-document. The SRD 5.1 is
+> licensed under the Creative Commons Attribution 4.0 International License
+> available at https://creativecommons.org/licenses/by/4.0/legalcode.
+
+The catalog contains names, spell levels, and class-list membership only. It
+does not reproduce spell descriptions. Canonical spell names retain the
+punctuation used by the source lists. The package lookup normalizes common
+apostrophe variants, case, and repeated whitespace without changing those
+display names.