fix: collapse orphaned wide-char lead on trailing-column overlay

[natemoo-re]

• Fixes 🐛 Overlay onto a wide char's trailing column is dropped #75
• Floating a narrow glyph onto the trailing column of an already-placed wide char dropped the overlay: the present loops advanced past the trailing column, so the wide char painted over both cells and the glyph was never emitted
• present_cups and present_lines now detect a partial overwrite (a wide char whose trailing back-buffer cell holds a non-space glyph), collapse the orphaned lead to a space at width 1, and let the overlay glyph emit on the next column
• Symmetric change in both present paths; rows without overlapping writes on wide chars stay byte-for-byte identical; cleanup only triggers on a detected overwrite
• The regression lives in test/width.test.ts under describe("wide characters") rather than a standalone file. fix: cluster base + U+FE0F as a width-2 emoji glyph #84 also adds to test/width.test.ts (describe("emoji width")), so whichever merges second needs a trivial rebase to keep both describes
• Test plan: deno test green (8 passed); deno lint + deno fmt --check clean

[pkg-pr-new]

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npm i https://pkg.pr.new/clayterm@86

commit: c12cd67

[codspeed-hq]

Merging this PR will degrade performance by 54.46%

❌ 1 regressed benchmark
✅ 22 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	long input burst (200 bytes)	576.1 µs	1,265 µs	-54.46%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing nm/repro/wide-char-overlay (c12cd67) with main (48ff4bb)}