feat(lang): L7 memory is safe unless it says unsafe

The typed buffers are slices: words/floats/fixeds/doubles/pointers(n) make
zeroed, bounds-checked []int/[]float/... and the type names mean them. buffer(n)
is a []byte, with text_of, Fs.read_bytes/write_bytes and view(xs, start, n).
bytes(), indexing a raw pointer or bytes, free, resize, Memory.*, raw file calls,
data_of and C externs are refused outside unsafe { } / unsafe function, and a
project's own files may write unsafe only with --unsafe; the runtime and packages
are the platform. A slice passed to an extern goes as its data.

What the change found: Sync's atomics on a slice header, words(n) uninitialised,
input's fixed axes in ints, truetype's fixed outlines as ints, skin matrices
typed int, gl_shader's source table made from raw bytes. render3d gets safe
entry points (safe_api.ludic). Rendering is byte-identical; a frame costs the same.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

selfhost/main.ludic

@@ -147,6 +147,7 @@ entry {
     else if a == "--windowed" { want = 1 }
     else if a == "--headless" { want = 2 }
     else if a == "--emit-llvm" { emit_ir = true }
+    else if a == "--unsafe" { g_unsafe_all = true }      # L7: this program's own files may write `unsafe`
     else if a == "--emit-module" { g_emit_module = true;  emit_ir = true }   # issue #64: prebuilt binary module IR
     else if a == "--fmt" { fmt = true }
     else if a == "--save-temps" { save = true }