feat(gasm): audit a .s corpus with audit-instructions --corpus

Assisted-by: GLM 5.3 Flash

docs/CLI.md

@@ -338,7 +338,7 @@ add: 16 bytes, args=24, frame=0 NOSPLIT
 ## audit-instructions
 
 ```text
-Usage: gasm audit-instructions [amd64|arm64|riscv64|loong64]
+Usage: gasm audit-instructions [--corpus [dir]] [amd64|arm64|riscv64|loong64]
 ```
 
 Compare the gasm encoder for the given architecture (default amd64) against the
@@ -361,6 +361,30 @@ gasm encodable: 580   go tool asm recognized: 1542
 shared: 580
 ```
 
+With `--corpus` the audit changes shape: it assembles every `.s` file under
+DIR (default `GOROOT/src`) with the gasm encoder only, no toolchain probing.
+A file whose name carries a recognisable `_arch` suffix is attempted for that
+architecture; a file without one is attempted for all four, exactly as a
+`GOARCH` build would compile it. The report gives the headline number (files
+that assemble for every target architecture), the per-architecture pass rates
+and the most common failure reasons with one representative file each, which
+drive the encodability backlog by frequency rather than by table order. A run
+over GOROOT takes under a second.
+
+```sh
+gasm audit-instructions --corpus
+gasm audit-instructions --corpus "$(go env GOROOT)/src/crypto"
+```
+
+```text
+corpus /usr/local/go/src: 627 files (365 generic, attempted for all architectures)
+  assemble for every target architecture: 108 (17.2%)
+  amd64: 77/464 attempted
+    148  instruction not encodable
+         e.g. /usr/local/go/src/cmd/asm/internal/asm/testdata/386enc.s
+...
+```
+
 ## scaffold
 
 ```text