fix: restore the wasm build on the current toolchain

[bokuweb]

Problem

cargo build -p wasm --target wasm32-unknown-unknown fails to link:

rust-lld: error: undefined symbol: keydown
rust-lld: error: undefined symbol: rx
rust-lld: error: undefined symbol: tx
rust-lld: error: undefined symbol: elapsed_us

Rust 1.82 stopped implicitly resolving undefined symbols in an extern "C" block to imports from the env module on wasm32-unknown-unknown. The build worked while rust-toolchain pinned 1.80.0 and broke as soon as the pin moved to 1.96.1 in #18, but nothing noticed because CI only builds the host target and wasm/out.wasm is a prebuilt artifact committed to the repo.

Changes

• Annotate the extern block in wasm/src/lib.rs with #[link(wasm_import_module = "env")]. This matches what worker.js supplies and what the wasm-opt --pass-arg=asyncify-imports@env.keydown step in the README already assumes.
• Rebuild wasm/out.wasm. The deployed playground was still running a binary from before fix: make vi (and other full-screen apps) usable, plus assorted CPU/bus correctness fixes #18, so it did not have the DIV, x0, out-of-range access or 8250 divisor latch fixes. The rebuilt module also drops from 5.1 MB to 3.5 MB.
• Build both wasm targets in CI so a toolchain change cannot silently break them again.
• wasm32-wasi was removed from the toolchain, so the WASI instructions in the README now use wasm32-wasip1.

Testing

• cargo build -p wasm --release --target wasm32-unknown-unknown and cargo build -p wasi --release --target wasm32-wasip1 both succeed on 1.96.1.
• Served the rebuilt wasm/ directory locally, booted Linux in the browser, logged in and opened vi /etc/passwd. The full-screen UI draws correctly.
• The stray m and ` characters that the currently deployed build prints around the printk: console [ttyS0] enabled lines are gone, which confirms the divisor latch fix is now in the playground binary.

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