feat(diff): composite — merge footnote/endnote content edits across reviewers (NoteOps composition)

[JSv4]

Follow-on from the DocxDiff N-way consolidate work (PR #228).

Current behavior

The composite merger does not compose note-scope (footnote/endnote) diffs across reviewers. A reviewer's edit to footnote/endnote content is not merged into the consolidated output. Base notes survive (a reviewer's body edits land, base footnotes are preserved on accept — fixed under PR #228), and a Debug.Assert tripwire in IrCompositeMarkupRenderer guards against a silent note-diff drop (the merger always produces NoteOps == null today). So this is a missing capability, not silent data loss.

The two-way DocxDiff engine already diffs note scopes (IrEditScript.NoteOps, footnotes then endnotes, mirroring WmlComparer.GetRevisions); the composite path simply doesn't merge them yet.

Desired

Extend the composite merger to compose per-note diffs across reviewers — analogous to body composition:

• Disjoint note edits by different reviewers compose; same-span note edits conflict (per ConflictResolution policy).
• Attribute each note revision to its contributing reviewer.
• Render the composed notes (remove the NoteOps-null tripwire once implemented) and represent them in the consolidated revisions + edit-script JSON.

Pointers

• Docxodus/Ir/Diff/IrCompositeMerger.cs (currently builds IrCompositeScript with NoteOps = null)
• Docxodus/Ir/Diff/IrCompositeMarkupRenderer.cs (the Debug.Assert(script.NoteOps is null …) tripwire to remove)
• Two-way precedent: IrEditScript.NoteOps, IrNoteDiff, the note-scope rendering in IrMarkupRenderer.
• Design context: docs/architecture/ir_diff_engine.md § N-way composite / Consolidate (v1 limitation list).

Acceptance criteria

• Two reviewers editing footnote/endnote text → composed + per-reviewer attributed in the consolidated notes; same-span note edits recorded as a conflict.
• reject ≡ base (including notes) under all policies; no silent loss; the NoteOps-null tripwire is removed.
• Regression: base 2-way parity stays 179/179; full suite green.