zed-extension/ was TOML-only (grammar + highlighting), so it couldn't
launch a language server. Turned it into a real Rust WASM extension
crate (crate-type = cdylib, zed_extension_api 0.7.0, wasm32-wasip2)
implementing language_server_command, registered via
[language_servers.strata-ls] in extension.toml.

strata-ls is located via worktree.which("strata-ls") on the user's
PATH — no binary download/bundling, consistent with this being a
local reference extension rather than a marketplace one. Missing
strata-ls surfaces a clear error through Zed's language server logs
rather than failing silently.

Verified: cargo build --target wasm32-wasip2 from zed-extension/
compiles clean; cargo build/test --workspace from repo root are
unaffected since zed-extension/ sits outside the crates/* glob and
has its own empty [workspace] table.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>