fix(renderer): resolve grapheme clusters by source row when scrolled

[bilby91]

Problem

Multi-codepoint grapheme cells (grapheme_len > 0 — emoji, variation-selector symbols, combining sequences) render the wrong glyph while the viewport is scrolled up, surfacing as stray ? / mismatched glyphs over scrolled content that appear and disappear depending on scroll position. Plain single-codepoint text and box-drawing are unaffected.

Root cause

In lib/renderer.ts, the render loop fetches each line with a scroll-adjusted row index:

• scrollback portion → getScrollbackLine(scrollbackLength - floor(viewportY) + y)
• visible portion → buffer.getLine(y - floor(viewportY))

But renderCellText looked the grapheme cluster up via getGraphemeString(y, x) using the raw on-screen row y. getGraphemeString resolves against the live active grid (terminal.ts wires it straight to wasmTerm.getGraphemeString), so once scrolled the two indices disagree by the scroll offset and a cell is painted with a grapheme from a different line. Single-codepoint cells render from cell.codepoint, which is why only special glyphs were affected.

Fix

Thread a graphemeRow through renderLine → renderCellText that tracks the line source:

• not scrolled / visible-screen portion → the scroll-adjusted screen row → correct grapheme
• scrollback portion → -1; scrollback graphemes are not addressable via the active-grid getGraphemeString, so fall back to the cell's base codepoint (eliminates the wrong-glyph artifact)

Known limitation

For graphemes that have scrolled fully into scrollback, the base codepoint is rendered, so a multi-codepoint cluster loses its combining marks (e.g. é → e). This is strictly better than the current wrong-glyph/? behaviour. Full fidelity would require a WASM API to fetch graphemes by absolute scrollback index (e.g. getScrollbackGrapheme) and have getScrollbackLine cells carry their cluster — left as follow-up.

Tests

• Adds lib/scrollback-grapheme.test.ts, which drives the real CanvasRenderer (mocking only the IRenderable/IScrollbackProvider interfaces), spies on fillText, and asserts a grapheme survives a scroll. It fails on main (draws the wrong-row sentinel) and passes with this change.
• Full unit suite: bun test lib/ --timeout 30000 → 435 pass / 0 fail.
• tsc --noEmit and Prettier clean.

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