fix: detect terminal resize on Windows via console buffer polling

[meszmate]

Problem

Fixes #114. On Windows, checkResize() was a stub that always returned false, so Program.tick() never entered the resize branch — context.width/context.height stayed frozen at startup values and the window_size message was never dispatched. zigzag TUIs on Windows never adapted to a resized terminal window.

Fix

Detect resize by polling GetConsoleScreenBufferInfo and comparing the srWindow dimensions against the last observed size.

WINDOW_BUFFER_SIZE_EVENT is intentionally not used: per the Windows Console docs, those records are only queued when ENABLE_WINDOW_INPUT is set on the input mode, which enableRawMode deliberately avoids (it wakes the stdin wait-handle without producing ReadFile bytes). Polling is the only viable mechanism under the current input mode, and it works under the classic console host. Under ConPTY/Windows Terminal a transiently stale reading self-corrects because the size is re-polled every frame; it degrades gracefully (returns false) when stdout isn't a console.

Notes:

• A first-read guard establishes the baseline so startup doesn't report a spurious resize.
• checkResize now takes the same stdout handle getSize uses (state.stdout_handle), so detection and measurement agree when a custom Config.output handle is set.

Verification

• Compiles clean for Windows (x86_64-windows-gnu), POSIX (native), and WASM.
• zig build test passes.