feat(tty): add composable virtual surfaces and PTY sessions

[wolfy-j]

Summary

• add canonical process-scoped TTY port and retained surface contracts
• add virtual viewport brokering with capability-bound grants and exclusive presentation leases
• expose compact Lua canvas and viewport APIs plus exec.Process:attach_terminal()
• model child allocation explicitly as typed PTYOptions / Lua pty = {...}
• connect real PTY lifecycle, resizing, input, cursor state, mouse, and paste for native and Docker executors
• separate focus from visibility so shells can choose live, frozen, minimized, or background presentation policy
• include one small two-process integration proof under tests/tty-surface-demo

Runtime properties

• zero idle polling and coalesced surface updates
• bounded canvas allocation and terminal scrollback
• exactly-once surface lease closure with closing-state exclusion
• graceful PTY shutdown followed by bounded forced termination
• UI/window policy remains outside the runtime core

Validation

• make lint — 0 issues
• go test ./...
• targeted go test -race across terminal API, Lua TTY/proxy modules, native exec, terminal services, and system TTY
• integration-proof lint — 2 entries, no issues
• git diff --check