W7f: sweep dead/legacy/fallback leftovers after native-pipeline removal

Post-deletion cleanup (units run only on wasm):
- types.h/compiler.cpp: drop the native-era SharedUnit fields so_name,
  bin_file_name, and the opt_so_optional cache-mode plumbing (no native
  optional .so path remains). The per-unit compile lock is re-keyed from
  so_name+.lock to wasm_name+.lock (still per-unit).
- unit_info() and to_string(SharedUnit*) no longer expose .so artifact fields.
- backend.h: drop the stale "+ fallback-token gate" comment.
- Docs/comments corrected to wasm-only reality: README, tests/README,
  site/doc C++ preprocessor + error_pages + unit_info pages, site/info intro,
  site/demo/unit-browser artifact card; the Phase-5 native-vs-wasm benchmark
  harness (tests/wasm_benchmark.py) reframed for the wasm-only backend.

Audit confirmed no live references remain to so_handle, load_shared_unit,
compiler_load_shared_unit, compiler_invoke*/_cli/_websocket/_serve_http,
COMPILE_SCRIPT/COMPILE_WASM_UNITS, or the native export-symbol constants;
request_ref_handler/dv_call_handler are kept (live wasm funcref casts).

Swept via the pi agent (delegated to a gpt-5.3-codex-spark sub-model);
independently re-verified on the host: run_cli_tests --include-wasm-kill =>
87 passed, 0 failed, 0 skipped.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

site/info/index.uce

@@ -166,7 +166,7 @@ RENDER(Request& context)
 						<h1>Make dynamic websites like it's 2006.</h1>
 						<p class="hero-lead">
 							UCE is a deliberately direct web runtime: files are pages, templates live next to control flow,
-							nginx fronts the whole thing, and a request can compile into a shared object on first hit.
+							nginx fronts the whole thing, and a request can compile into a wasm module on first hit.
 							It is built in the image of PHP's non-architecture, but with an explicit request object,
 							component rendering, and a built-in WebSocket broker.
 						</p>
@@ -177,7 +177,7 @@ RENDER(Request& context)
 						</div>
 						<div class="signal-grid">
 							<? render_signal("Files are routes", "A .uce page is the thing you edit and the thing nginx serves."); ?>
-							<? render_signal("Compile on demand", "First request builds a shared object, later requests reuse it."); ?>
+							<? render_signal("Compile on demand", "First request builds a wasm module, later requests reuse it."); ?>
 							<? render_signal("WebSockets included", "Same runtime, same page path, broker-managed connection state."); ?>
 						</div>
 					</div>
@@ -314,7 +314,7 @@ RENDER(Request& context)
 					</details>
 					<details>
 						<summary>What makes it different from old PHP?</summary>
-						<p>The request object is explicit, components and named handlers are native, markdown support is built in, and WebSockets use a broker-owned connection model inside the runtime.</p>
+						<p>The request object is explicit, components and named handlers are first-class, markdown support is built in, and WebSockets use a broker-owned connection model inside the runtime.</p>
 					</details>
 				</section>
 			</main>