feat(asm): emit linkable ELF and Mach-O objects with external symbols

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docs/ARCHITECTURE.md

@@ -243,10 +243,17 @@ File-level assembly (`AssembleFile`) goes beyond single functions: it
 materialises the file's static symbols (`GLOBL`/`DATA`) in a data section
 behind the code and resolves references to them (`mask<>(SB)`) to
 RIP-relative loads whose displacements point inside the resulting image, so
-the bytes are self-consistent at any base address.  External (non-file-local)
-symbols are rejected: they need object-file emission, which — together with
-the remaining EVEX forms and the other architectures — is the rest of
-Phase 2.
+the bytes are self-consistent at any base address.  References to symbols no
+`GLOBL` defines are kept as relocations on the function layout, and the
+object-file emitters turn the whole image into a linkable object: the ELF
+and Mach-O writers (`gasm asm --format elf|macho`) lay the code and data out
+as `.text`/`.data` (or `__text`/`__data`) sections, export a symbol per
+`TEXT` and `GLOBL` (the `<>` ones local, the rest global) and emit one
+PC-relative relocation per static-symbol reference — undefined external
+symbols included, so the output links with the system toolchain.  GOOBJ
+emission, the format the Go linker consumes directly, is the remaining
+piece of Phase 2 (together with the rest of the EVEX set and the other
+architectures).
 
 ## Extension points