fix(renderer): keep kitty image placeholders (U+10EEEE) out of OSC 66 text-sizing

[arthurkatcher]

Summary

Fixes #1300.

When explicit_width is enabled, both diff-emit paths wrapped every pooled grapheme in OSC 66 text-sizing, including U+10EEEE, the kitty graphics protocol's Unicode placeholder. kitty only recognizes plain U+10EEEE cells as image placements, so wrapped placeholders render as blank cells exactly where a transmitted image should appear. kitty >= 0.40 answers the text-sizing probe, so explicit_width is on by default in real kitty sessions and the placeholder flow is broken out of the box.

This change skips explicitWidthOutput for graphemes that start with U+10EEEE (bare or carrying row/column diacritics) in both call sites in renderer.zig. All other graphemes keep explicit-width output.

Verification

• Regression test first, per AGENTS.md: renders an emoji plus a placeholder row with explicit_width = true through TestRenderer and asserts the emoji is still OSC 66 wrapped while the placeholder cells go out bare. Red on main (fails on the wrapped-placeholder assertion), green with the fix.
• bun run test:native from packages/core: 1709/1715 passed, 6 skipped, 0 failed.
• End to end: the patched build driven under a pty harness that answers capability probes exactly like kitty 0.46.2 (xtversion, DECRQM, text-sizing probe cursor reports, kitty graphics query, DA1), so explicit_width is genuinely enabled through detection rather than forced. Captured output shows the control emoji wrapped in ]66;w=2; while all placeholder cells are emitted bare; before the patch every placeholder cell was wrapped.
• Real-world origin: found in an app running in kitty 0.46.2 where images transmitted with a=T,U=1 and placed via placeholder cells rendered as blank space; with this change kitty draws them correctly.

zig fmt clean on both files.