feat(core): add embedded terminal runtime

[jlongster]

Problem

PR #1317 pins Ghostty and makes its VT module available to OpenTUI, but OpenTUI still has no owned terminal state or mode-aware input boundary. The previous runtime PR (#1307) proved the behavior through a dynamic C loader; that implementation no longer matches the merged static Zig-module architecture.

This PR rebuilds only the internal runtime layer on the pinned Ghostty module. It deliberately excludes OpenTUI composition, native handles/FFI, TypeScript, and the public renderable so terminal ownership and input semantics can be reviewed independently.

Changes

• Own a Ghostty Terminal and persistent TerminalStream across the complete runtime lifetime.
• Preserve parser state across split CSI, OSC, and UTF-8 writes.
• Bound parser-driven allocations at 1 MiB and report processing failures.
• Support resize, viewport scrolling, keyboard, mouse, paste, and focus encoding from current terminal modes.
• Suppress SGR-pixel mouse reports because OpenTUI currently receives cell coordinates rather than physical pixels.
• Capture terminal-generated PTY responses with a 1 MiB bound, partial drains, and recoverable overflow reporting.
• Preserve already queued responses when later output exceeds the bound.
• Use growable owned encoder output rather than fixed native buffers.

Scope

This is the internal adapter/runtime layer. It does not add native handles, C/TypeScript FFI, OptimizedBuffer composition, cursor projection, EmbeddedTerminalRenderable, screen observation, benchmarks, or examples.

The next stack layer will port the proven compositor and renderable behavior from closed PR #1308 onto this runtime.

Testing

• Focused embedded-terminal native suite (6 passed)
• bun run test:native (1873 passed, 6 skipped)
• bun run fmt:check
• bun run lint