adds ENDF-manual-backed reference lookups

[tokamaster]

I converted the ENDF-6 Formats Manual (2018) to Markdown using docling and passed it to Codex. Probably I should have used the 2023 manual, but this is just a test to see if the combo docling + Codex is actually effective.

Summary

Adds offline, manual-backed lookup support for common ENDF-6 reference codes while keeping the extension lightweight. The change focuses on curated hovers and completions rather than introducing a parser or language server.

User-Facing Improvements

• Richer hovers for MF, MT, NSUB, NLIB, INT, and LR references in descriptive text.
• Hovers for ENDF record type names including TEXT, CONT, HEAD, LIST, TAB1, TAB2, INTG, SEND, FEND, MEND, and TEND.
• Completion suggestions after keys such as MF=, MT=, NSUB=, NLIB=, INT=, and LR=.
• Existing fixed-column payload, tail-field, status-bar, and numeric parsing behavior is preserved.

Implementation Notes

• Adds endfReference.js as the shared curated lookup source.
• Updates extension.js to use the shared lookup helpers for hovers and completion items.
• Adds a small Node test suite under test/endfReference.test.js.
• Documents the new behavior in README.md and CHANGELOG.md.

Tests Run

• npm test
• node --check extension.js
• node --check endfReference.js

Known Limitations

• This is a core-table pass, not a complete encoding of Appendix B.
• The extension still does not validate ENDF structure, sequence numbers, ordering, or record-specific payload semantics.
• There is no outline view, command-palette lookup, diagnostics engine, parser, or language server in this change.

[tokamaster]

not sure about having tests... these require at least one .endf file and I don't want to make the tests dependent on any. this one is using Li7 because it is the test file I have locally.

[tokamaster]

do we need completion? probably not. I think this just complicates the extension, as it is not meant to be used for writing .endf but just inspecting/reading them.