fix(render): unwrap single-cell tables that wrap a nested table

[mathurshubham]

Fixes #14.

This implements direction (3) from the issue — the narrow single-cell-wrapper
case — since it covers the reported form shape without touching genuine
nested grids or costing the clean-Markdown property.

What

A nested table inside a cell flattens to a / b runs. For real multi-cell
grids that's unavoidable in GFM, but when the outer table is a single
cell it is pure layout scaffolding (the Word-form shape: one wrapper table
around the actual content), and flattening collapses the inner grid into one
enormous cell for no benefit.

The renderer already unwraps trivial Layout-kind single-cell tables
(render/markdown/mod.rs); no frontend currently produces Layout, so the
mechanism sat unused. This PR extends that arm: a single-cell table whose
lone cell holds a nested table unwraps to its blocks regardless of kind,
letting the inner table render as a real grid.

Deliberately unchanged:

• plain | x | 1×1 data tables stay tables (existing
  data_table_1x1_not_unwrapped test still passes untouched);
• a nested table inside a multi-cell grid keeps today's flatten (new test
  pins it) — that's option (1)/(2) territory and out of scope here.

Wrapper-in-wrapper unnests recursively, so multi-level form scaffolding
resolves too.

Before / after

The issue's repro (3×2 table inside a single-cell outer table), reproduced
byte-for-byte on main and converted with this branch:

-|  |
-| --- |
-| Section A<br>Metric / Value<br>Height / 120<br>Weight / 34<br>End of section |
+Section A
+
+|  |  |
+| --- | --- |
+| Metric | Value |
+| Height | 120 |
+| Weight | 34 |
+
+End of section

Because the fix lives in the renderer, it applies to every format that can
produce nested tables (docx, rtf, epub/html), not just docx.

Tests

• Renderer unit tests: wrapper around a nested table unwraps (the repro
  shape); nested table in a multi-cell grid still flattens; wrappers unwrap
  recursively; plain 1×1 data table unchanged.
• New handmade fixture handmade-nested.docx (single-cell wrapper with a
  nested 3×2 table, plus a two-cell table with a nested table to pin the
  flatten path) + its snapshot.
• cargo fmt --check, clippy --workspace --all-targets --all-features -D warnings, cargo test --locked all green; no existing snapshot
  changed — the only snapshot in the diff is the new fixture's.

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