ci(wasm): pin wasm build toolchain for deterministic size; re-tighten size budget

[dean0x]

Problem

The WASM size guard in ci.yml is non-deterministic. The wasm job uses unpinned dtolnay/rust-toolchain@stable, so codegen size drifts as Rust stable advances. During the v0.3.0 release CI, crates/mds-wasm/pkg/mds_wasm_bg.wasm measured 601,566 bytes — over the 600,000 guard — with no WASM source change (local toolchain emits ~592,494 bytes). The binary sits right on the threshold and the CI toolchain tips it over.

Band-aid applied

Raised the guard to 650,000 (ci.yml, landed on main in the v0.3.0 release PR #126) to unblock. This trades away bloat sensitivity.

Proper fix

1. Pin the wasm build toolchain (and ideally the napi build toolchain) to a known Rust version so binary size is reproducible across local/CI/time.
2. Once deterministic, re-tighten the size budget to a meaningful value with documented headroom.

Found during the v0.3.0 release.