remove deprecated rust artifacts and elvm bytecode spec

The Rust bootstrap was archived in 4f3543b and removed from the working
tree in e7a49eb. The bytecode tier was retired in 9fca4dc. What remained
on disk was leftover platform binaries (dist/platform/el-macos-universal,
el-windows-x86_64.exe) that nothing should be invoking, the elvm.md spec
for the retired bytecode tier, and the 8.7GB target/ build cache that
was tracked despite being in .gitignore.

Untracks target/, removes the platform binaries and elvm.md, and updates
spec/language.md so its self-hosting section no longer references the
genesis Rust path. The canonical toolchain is dist/platform/elc against
el-compiler/runtime/el_runtime.{h,c} — one compiler, one runtime, one
language.

spec/language.md

@@ -671,7 +671,7 @@ The El compiler lives in `el-compiler/src/`:
 - `codegen.el` — C emitter
 - `compiler.el` — pipeline wiring + `main()` entry
 
-These are concatenated into `elc-combined.el` (single-file bootstrap edition). The genesis Rust compiler at `target/debug/el` was used once to produce the first self-hosted compiler binary at `dist/platform/elc`. From that point forward, `elc` compiles itself and all El programs.
+These are concatenated into `elc-combined.el` (single-file bootstrap edition). The bootstrap compiler binary lives at `dist/platform/elc`; from there `elc` compiles itself and all El programs.
 
 ### 13.3 C Runtime