runtime: complete PTY and child-process behavior for full OpenCode shell, LSP, and formatter paths

[proggeramlug]

Parent: #8506
Related completed work: #6563
Related open FFI work: #6562

Problem

The full OpenCode target exercises more than basic PTY spawn. It uses PTYs for interactive sessions and process APIs for shell tools, language servers, formatters, signals, cancellation, and streaming output. OpenCode has both bun-pty and @lydell/node-pty backends.

The goal is one native Perry PTY/process implementation exposed through the compatible import shapes, with bun-pty also becoming available through the generic FFI work where appropriate.

Scope

• PTY spawn, write, resize, data, exit, kill, cwd, env, and terminal dimensions
• Unix PTY and Windows ConPTY lifecycle
• child-process stdio pipes, backpressure, exit/error ordering, signals, cwd/env, and cancellation
• long-running LSP process lifecycle and formatter timeout behavior
• no N-API or JavaScript-runtime fallback

Acceptance

• native PTY echo/resize/signal fixtures pass on supported platforms
• OpenCode interactive terminal sessions work
• representative LSP startup/shutdown and formatter executions pass
• shell tool cancellation kills the correct process tree
• tracking: compile and run the complete OpenCode source graph without feature stripping or a JS runtime #8506 can exercise these paths without runtime JS