w6c — amd64 compiler. Reads `.ww`, writes Plan 9 amd64 assembly (`.s`).

- Plan 9 C style: tabs, K&R, short names. Mirror `ref/plan9front/sys/src/cmd/8c/` and `ref/plan9front/sys/src/cmd/cc/`. Do not invent new data shapes — reuse `Prog`/`Adr`/`Node`/`Sym`/`Type` conventions.
- Frontend (lex, parse, check) lives in `../wcc/` and links as `libwcc.a`. This directory owns codegen only.
- `gc.h` is the per-target header (Plan 9 cc convention). `cgen.c` performs the current AST lowering into a `Prog` list, including switch lowering; `txt.c` emits the assembler text. There is no peephole or register-allocation pass yet.
- The text output must be readable by `w6a` (the amd64 assembler under `../w6a/`). When in doubt about syntax, run `w6a` on the output.