Fix deletion of footnote and endnote references

[JSv4]

Deleting a footnote or endnote marker now removes its native definition and reference together, preserves surrounding text, and supports one-step undo/redo. Selections retain note identities separately from text offsets; browsers without deletion target ranges use the adjacent DOM reference. Note-only paragraphs remain editable.

Closes #44.

Validation:

• Confirmed the native-note deletion regression failed before the fix; added parameterized caret/selection cases, save/reopen, mixed-note boundaries, empty paragraphs, and missing/empty browser target ranges.
• Lint, TypeScript, 82 unit tests, library/demo builds, API checks, and packed-consumer checks pass.
• Full browser suite: 63 passed; 2 optional NVCA stress/performance tests skipped by the existing configuration.
• Adversarially reviewed with gpt-5.6-sol; reproduced and fixed its compatibility finding, then received a clean re-review.