docs(footnote): sd-2656 plan + implementation report

End-to-end documentation for the footnote rendering fidelity epic:

  docs/superdoc-feature-reports/sd-2656-plan.md
    Original implementation plan: ticket inventory across the epic,
    OOXML grounding (§17.11), code surface map with line numbers,
    surgical approach for each slice, RED test scaffolds, falsifiable
    success criteria.

  docs/superdoc-feature-reports/sd-2656-implementation-report.md
    What shipped, with measurements:
      - Harvey NVCA: 57 -> 53 pages (Word baseline 51, +5%)
      - pnpm test:layout vs superdoc@1.32.0:
          535/543 docs (98.5%) byte-identical
          5 unique-change docs, all NVCA-style footnote-rich legal
          templates (the intended scope)
      - pnpm test:visual: "no visual differences found"
      - 16,649 unit tests across 5 packages, all green
    Slice-by-slice walkthrough (SD-3049 / 3050 / 3051 / 2986/B1+B2 /
    2658 / 2662), architecture compliance (Guard C parity test),
    pr-reviewer findings + resolutions, deferred work, repro commands.

Refs: SD-2656

Author: tupizz