test(pty): exercise native ConPTY runtime

[jlongster]

P1 — native ConPTY runtime

Adds a reusable self-spawned Rust console fixture and six direct TerminalService tests on Unix and Windows, without daemon/client transport. Covers actual child stdin/stdout, Unicode, cwd/env/argv, supported child-PATH lookup, console resize, snapshots/replay, terminal replies, and independent terminals.

• portable-pty 0.9.0 unchanged; existing ConPTY runtime passes the basic tests without runtime rewrites. Actor-owned Ghostty and one PTY reader/writer preserved.
• Minimal windows-sys 0.61.2 Console dev dependency; native Windows x64/ARM64 CI adds Clippy and runtime diagnostics.
• Fixed pre-existing Unix-only client imports exposed by Windows Clippy (coordinated with transport).
• Fixture explicitly restores blocking mode after BSD accept; its deadline bounds hung tests only, not runtime cleanup.
• Optional relative-program lookup probe is outside Git; no undocumented cross-platform resolver parity imposed.

Verified at 3a1d57d

Windows run 33777172486:

• Native x64: 19 library + 5 callback + 6 real ConPTY runtime tests passed; standalone library 19/19.
• Native ARM64: same 30 tests passed, plus standalone library 19/19. OS architecture and PE target assertions passed; not x64 runtime emulation.
• Both: formatting, Clippy all targets/features, build and executable smoke passed.

Core run 33777172540:

• Linux: 51 tests passed. macOS: 47 tests passed.
• Miri: 5 Stacked + 5 Tree Borrows passed.
• The one ignored runtime test is the deliberately self-spawned fixture entry, not omitted coverage.
• Local Linux full suite and play query-response smoke also passed.

P1 basic native runtime gate is complete. P2 termination/EOF/cleanup follows as a separate stacked PR. Named-pipe transport is an independent lane. Do not merge.