feat(core): add embedded terminal renderable

[jlongster]

Problem

The runtime layer in #1338 owns Ghostty terminal state and input semantics, but it cannot render into OpenTUI or participate in layout, focus, cursor, mouse, and portable TypeScript FFI. The previous renderable PR (#1308) proved this behavior against a dynamic C loader and an older OpenTUI native surface.

This PR ports that user-facing layer onto the pinned Ghostty Zig module and current OpenTUI handle/Node/Bun architecture.

Changes

• Translate Ghostty RenderState dirty rows, colors, styles, graphemes, wide cells, and cursor state directly into OptimizedBuffer.
• Preserve Ghostty’s authoritative terminal-cell widths through a native drawGrapheme operation.
• Recover from partial render-state allocation failures by discarding incomplete retained state and forcing a rebuild.
• Add kind-11 embedded-terminal handles with stale, wrong-kind, and idempotent-destroy coverage.
• Export a fixed-width native ABI for lifecycle, composition, cursor, scrolling, input encoding, and PTY responses.
• Keep exported symbols available on unsupported custom targets while returning an explicit unsupported status.
• Add portable Bun/Node FFI with direct borrowed typed-array arguments, bounds validation, and required-size key output retry.
• Add EmbeddedTerminalRenderable with focus, Kitty releases, paste, mouse forwarding, scrollback fallback, resize propagation, hardware cursor ownership, and cleanup.
• Use Ghostty’s W3C physical-key mapping instead of copied enum numbers.
• Suppress SGR-pixel reports because OpenTUI currently has terminal-cell rather than physical-pixel coordinates.

Scope

This is the second layer of native GitHub Stack #1341 and targets #1338. It excludes PTY/process ownership, screen snapshots/change notifications, benchmarks, examples, selection/search, and child-terminal graphics.

Testing

• Focused native embedded-terminal suite (10 passed)
• Focused Bun renderable and native-handle suites (18 passed)
• Full native suite (1878 passed, 6 skipped)
• Embedded-terminal Node suite (8 passed)
• Packed Node/Bun distribution smoke passed
• Unsupported x86_64-freebsd custom-target build passed
• bun run fmt:check
• bun run lint

The full Node lane reached one unrelated ALSA audio-device failure after all embedded-terminal tests passed. The full Bun lane reached the embedded-terminal tests successfully before an unrelated Bun runtime crash in the existing diff suite.