Add readKeyTimeout primitive for non-blocking keystroke reads

[chapelsoftware]

Summary

Adds readKeyTimeout : ℤ → ⟨Bool, ℤ⟩ — a non-blocking keystroke read with
millisecond precision.

• ⟨⊤, keycode⟩ when a key arrives within the timeout
• ⟨⊥, 0⟩ when the timeout elapses first
• Option-shape matches stdlib/option.goth convention

Implementation uses libc::poll under #[cfg(unix)], matching the existing
rawModeEnter / rawModeExit precedent (Unix-only terminal primitives,
not_implemented elsewhere).

Motivation

The current readKey primitive blocks indefinitely on stdin, which makes
timer-driven TUI programs impossible: tickers, blinking cursors, animated
displays, updatetime-style features, ambiguous-keybind timeouts (vim's
jk escape), autosave ticks. Every such program needs a read-with-timeout.

docs/PROJECT-STATUS.md lists "TUI demo compilation (tui_demo.goth)" as
active work. This primitive is the smallest step toward enabling it.

Demo

examples/tui/timer.goth: a live elapsed-seconds counter. space pauses,
q quits. Without readKeyTimeout, readKey would block and the counter
couldn't tick.

Elapsed: 7s  [running] (space=pause, q=quit)

Design notes

• Argument unit is milliseconds: keeps the contract in the type with
  no hidden granularity footgun.
• Result shape matches Option (⟨Bool, α⟩ per stdlib/option.goth):
  reuses the project's existing vocabulary rather than introducing a new one.
• ◇io effect: the type signature's comment notes the ◇io annotation
  as aspirational — see "Parser note" below.
• Argument clamping: negative timeouts clamp to 0 (non-blocking poll);
  values above i32::MAX saturate.

Test plan

• Primitive returns ⟨⊤, keycode⟩ when a byte is piped in
• Primitive returns ⟨⊥, 0⟩ when stdin stays open but quiet for the timeout
• Zero-timeout returns immediately if input is ready
• Negative timeout clamps to 0
• cargo test --package goth-eval — 216/216 pass
• cargo test --package goth-check — 1/1 pass
• Regression-checked existing examples (pipeline, square, sha256) produce identical output
• Interactive TTY test of examples/tui/timer.goth

Parser note (separate finding)

While writing this I noticed stdlib/tui.goth does not currently parse. A
minimal reproducer:

╭─ foo : () → ()
│  ◇io
╰─ print "hi"

fails with Unexpected token: Some(Diamond), expected identifier. This
contradicts docs/EFFECT-SYSTEM-ROADMAP.md, which lists "Lexer/Parser: The
◇ token… recognized in function box middle lines" under "What Works".
Because of this, I intentionally did not add │ ◇io annotations to the
new primitive's stdlib wrapper and kept the demo using primitives
directly. Happy to submit a follow-up PR targeting the parser if useful.

Scope

No stdlib or interpreter behavior changes outside the new primitive and
its registration. No dependencies added (libc was already a target-cfg(unix)
dep). No new error variants.

🤖 Generated with Claude Code