refactor!: rename slhx to hemx

Rename the tracked product identity, crate/package names, Rust paths/macros, generated artifacts, runtime files, public attributes, examples, docs, requirements, and tests from slhx to hemx without compatibility shims.

Verified with cargo run -p hemx-xtask -- test, cargo test -p hemx-derive --test compile_fail, cargo test -p hemx-js, cargo test -p hemx-axum, cargo test -p hemx-v0-examples, cargo check --workspace, redgate list, redgate refs, redgate health --strict, git diff --check, and git grep/ls-files legacy-name audits.

req: misc/001

req: codegen/001

req: component/004

req: runtime/001

examples/techdemo/templates/partials/architecture_activity.heml

@@ -1,5 +1,5 @@
 <ol class="activity">
   <li>Clicked a real anchor</li>
-  <li>Fetched HTML with X-SLHX-Partial</li>
+  <li>Fetched HTML with X-HEMX-Partial</li>
   <li>Preserved native fallback semantics</li>
 </ol>