Fill test gaps: clipboard clamp, .cell absolute refs, :w! CSV overwrite

[garritfra]

Problem

A few hot paths and explicitly-documented invariants lack tests. Each is small but high-value because regressions would be silent.

Gaps

1. clipboard::adjust_formula clamp behaviour (crates/cell-sheet-tui/src/clipboard.rs:43–47) — negative row_delta / col_delta that would push refs below row/col 0 are clamped via .max(0), but this is untested. Off-by-one here silently mis-paste formulas.

2. .cell round-trip with absolute refs — crates/cell-sheet-core/tests/integration.rs covers basic formula round-trip but not =$A$1+$B$1-style absolute references.

3. :w! overwriting a .cell formula sheet with CSV — AGENTS.md calls this out as an invariant ("CSV export flattens formulas; the :w! force-save flow is what allows CSV to overwrite a formula sheet"). No test covers the path.

4. fn_min/fn_max empty-input semantics — both return 0.0 on empty input (intentional, but undocumented by tests). A future maintainer might "fix" it to be consistent with fn_average (which returns #DIV/0!).

Proposed fix

One PR adding ~6 small tests:

• adjust_formula_row_clamps_to_zero and adjust_formula_col_clamps_to_zero in clipboard.rs.
• cell_format_preserves_formula_with_absolute_refs in tests/integration.rs.
• TUI integration test for :w! CSV overwrite of a formula .cell (or document why this lives only in headless tests).
• min_empty_returns_zero and count_skips_text_and_empty in functions.rs.

Priority

MEDIUM — pure test additions, no risk, locks in current behaviour.