fix: width-aware input/layout, prompt paste, and FCmd mapper guard

[rorygraves]

Addresses the code-review findings.

Findings addressed

High — Prompt drops bracketed paste. Prompt.handleKey had no InputKey.Paste case and fell through to no-op. It now inserts the first pasted line at the cursor, mirroring TextField's single-line paste behaviour. (TextField and MultiLineInput already gained paste handling in #203, so only Prompt remained.)

High — Input viewport sized by char count, not cells. AnsiRenderer.visibleInput budgeted the fixed prefix and scrolled the editable suffix with length / take / slice, while cursor placement already used WCWidth. CJK/emoji text could overflow the declared lineWidth or scroll incorrectly. The prefix is now clipped and the suffix scrolled by rendered cell width, with new takeCells / charIndexAtCell helpers. For pure-ASCII input the math reduces to the previous behaviour (existing goldens unchanged). nodeExtents' auto-size fallback measures the prompt the same way.

Medium — Layout measured text by String.length. Layout.measureVNode now measures TextNode/InputNode width via WCWidth.stringWidth, so rows, columns, grids, fills, and hit-test zones place wide/combining text consistently with the renderer.

Medium — Cmd.FCmd success mapper unguarded. A throwing toCmd(result) died on the execution context, publishing neither the intended command nor an error and leaving the app stuck loading. The success branch now catches NonFatal and surfaces a TermFlowError.Unexpected, matching the existing failure path.

Medium — supplementary-plane emoji in the diff renderer: already resolved on main by the RenderCell.glyph / renderedGlyph plumbing (drawString stores the full glyph; the diff emits renderedGlyph). No change needed; noting for completeness.

Tests

• PromptSpec: paste inserts at cursor, keeps only the first line, empty paste is a no-op.
• AnsiRendererSpec: wide-char input viewport bounded to the declared cell width with the cursor landing correctly.
• LayoutSpec: measureVNode returns rendered cell width for CJK / emoji / combining text.
• TuiRuntimeSpec: a throwing FCmd success mapper surfaces a TermFlowError.

sbt ciCheck is green.

[codecov]

Codecov Report

❌ Patch coverage is 91.11111% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...een/src/main/scala/termflow/tui/AnsiRenderer.scala	87.87%	4 Missing ⚠️

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