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>

scripts/compile_wasm_unit

@@ -21,9 +21,9 @@ COMMON_FLAGS=(
 	--target=wasm32-wasip1
 	-fPIC -fvisibility=default -fvisibility-inlines-hidden
 	-O1 -g -std=c++20
-	# -w as in scripts/compile: warnings are not failures. The server captures
-	# this script's output and treats any non-empty result as a compile failure
-	# (then drops the .wasm), so a successful build must be silent.
+	# The server captures this script's output and treats any non-empty result as
+	# a compile failure (then drops the .wasm), so a successful build must be
+	# silent; warnings are intentionally suppressed.
 	-w
 	# must match the core build ABI: units with RTTI/EH enabled import
 	# typeinfo/unwind symbols the -fno-rtti/-fno-exceptions core cannot provide