fix(sheet): preserve merged-cell spans that extend past the used range

[mathurshubham]

Fixes #8.

What

A merge anchored on populated cells but reaching into empty rows or columns
was clipped to the used range, so a valid source merge (F1:O3 with only
F1 populated) silently collapsed to an ordinary 1×1 cell in
to_document().

The collapse happened in two layers, so both change:

1. Sheet frontend (src/formats/sheet/mod.rs) intersected each merged
   region with calamine's used range before building the grid — the clip the
   issue reports.
2. GridBuilder::finish (src/model/table.rs) dropped trailing rows
   holding only covered positions ("all-empty" treated covered slots as
   empty) and then re-clamped the surviving spans — so even an unclipped
   span would have shrunk back.

The fix

• The sheet frontend keeps a region's full extent. The two existing
  defenses are preserved unchanged: the clamped top-left still decides the
  origin, and regions that don't intersect the used range at all are still
  ignored (the merge_outside_the_used_columns_is_ignored test is
  untouched). Rows below the used range are appended so the overhang has
  positions to materialize into.
• finish()'s trailing trim now exempts span-claimed rows: covered
  positions are structure a producer declared, and dropping them silently
  shrinks a valid span. Rows of plain empty cells still trim (new test pins
  that).

Runaway regions stay bounded — a region whose full extent would blow the
expansion budget (a crafted A1:XFD1048576 merge, or a real whole-column
merge) degrades to today's clipped form instead of materializing: the file
still converts, it just loses the overhang (new test proves it).

Behavior

The reporter's exact openpyxl repro, before → after:

-1 1
-origin 1 1
+3 10
+origin 3 10

The rendered Markdown is unchanged — the renderer already trims the
trailing empty area visually, so to_markdown still emits
| Merged heading |. The fix is observable through to_document(), which
is the surface the issue is about. Consequently no existing snapshot
changed.

Tests

• Sheet: full-extent span over a 2×2 used range (5×5 merge), the issue's
  1×1-anchor shape (3×10 span, 29 covered), the existing out-of-range-merge
  defense unchanged, and the whole-sheet crafted merge degrading to the
  clipped form instead of erroring
  (oversized_merge_degrades_to_the_clipped_form).
• Model: trailing span-claimed rows survive finish() (replaces the test
  that asserted the old clamping behavior); trailing empty-origin rows still
  trim.
• Integration: new handmade-overhang.xlsx fixture + snapshot, and a
  model-retention test asserting the 3×10 grid and spans via
  to_document().
• cargo fmt --check, clippy --workspace --all-targets --all-features -D warnings, and cargo test --locked all pass; only the new snapshot
  was added, nothing else moved.

Note

This branch is rebased on current main and composes with the new
resolve_header_rows detection (fe97550) — header resolution runs on the
finished grid, including the preserved overhang rows.

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[mathurshubham]

Closing in favor of #13, which was opened first and takes the same two-layer approach (sheet-side extent + the finish() trailing-trim) — I hadn't seen it when I opened this.

One deliberate difference worth carrying over from this PR if useful: an integration fixture (handmade-overhang.xlsx) plus a to_document() retention test asserting the 3×10 grid and spans — happy to contribute those to #13 as a follow-up if wanted. Verified #13 against the issue's openpyxl repro locally; it resolves it.

[mathurshubham]

Reopening: #13, which this was closed in favor of, was closed when its author's account was deleted, so #8 is unaddressed again.

Since then I've folded in the one behavior where #13 had the better call — an oversized region (a crafted whole-sheet merge, or a real whole-column merge) now degrades to today's clipped form instead of failing the conversion with ResourceLimit (c19fbb0).

Branch is rebased on current main and the full suite passes.