test: add terminal behavior regression suite

[noodanee]

Summary

• add a Zig terminal behavior regression suite covering cursor movement, erasing/insertion, SGR, scroll margins, modes, soft reset, and OSC title handling
• make zig build test run both the existing terminal tests and the new behavior tests through a single test root
• fix two core behavior gaps found by the suite: alternate screen clearing with the current erase background and DECSTR reset of saved cursor/style state
• update the packaged WASM artifact after the Zig core change

Test Plan

• zig build test --summary all
• zig build -Doptimize=ReleaseSmall && diff -q zig-out/bin/wterm.wasm packages/@wterm/core/wasm/wterm.wasm
• pnpm test
• pnpm build
• pnpm type-check
• pnpm lint
• pnpm format:check
• git diff --check main...HEAD

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