fix(core): refresh dimensions after resize debounce

[dajiaohuang]

Summary

• refresh cached TTY dimensions when a debounced SIGWINCH is handled
• sample the refreshed dimensions when the debounce expires
• use stdin as the size source when it is the TTY and stdout is piped
• cover stale cached dimensions and piped-stdout setups with regression tests

Browser-hosted PTY bridges can deliver a resize signal before the runtime's cached stdout.columns and stdout.rows reflect the new kernel window size. The existing resize path captured those cached values immediately, so initial attach and shrink events could keep rendering at the previous width.

OpenTUI also always read dimensions from stdout, but some launchers pipe stdout while keeping stdin attached to the controlling terminal. In that setup the resize signal arrives, yet stdout has no terminal dimensions. The resize handler now refreshes and samples the active TTY stream after the debounce expires.

This keeps the existing debounce behavior and is related to anomalyco/opencode#42225.

Closes #1344

Testing

• bun run test:js (5,397 passed, 23 skipped)
• bun run test:js:node with Node.js 26.4.0 (4,664 passed, 6 skipped)
• bun run build:lib
• bun run fmt:check
• bun run lint
• Linux PTY reproduction covering initial attach, grow, and shrink