refactor: keep the front end, drop everything downstream of it

The milestone structure had stopped describing the compiler and started
shaping it: m7.c, check_m7.c, tests/m2..m9, and a checker and emitter that had
each grown past 2,500 lines because there was nowhere else to put anything.
Restart from the pipeline instead.

What is left is the front end -- lexer, parser, types, ownership, semantic
analysis -- and the fixtures that describe it. The C backend, the DOSBox-X
runner, the milestone registry and the batch build are removed. The driver now
stops after semantic analysis; a code generator attaches where emit_c did.

Fixtures move from milestone directories to what they check:

    parse/     grammar            own/       ownership and borrowing
    types/     type rules         optional/  optionals and error unions
    format/    formatting, try    units/     units and visibility
    generic/   generics           pending-backend/

pending-backend/ holds the three fixtures that can only be checked by running
a program -- that the bounds check traps, that --no-checks removes it, and that
drops and defers actually fire, verified through a fake allocator. Those are
not front-end tests and are not pretending to be; they come back first when
there is a code generator.

tests/run.py replaces the DOSBox-X harness. It builds the front end with the
pinned Watcom's Windows-hosted driver and runs every fixture in about two
seconds, and it does something the old runner structurally could not: it reads
the `// ERROR:line:text` marker each fixture carries and checks the diagnostic
against it. Those markers have been in the tree all along, unverified, because
DOS could not redirect the compiler's stderr and only the exit code was ever
compared.

133/188 pass. The 55 failures are not regressions -- they are what was already
true and invisible:

- units (27) and generic (23): `import`, `comptime` and generic declarations
  parse and are then dropped on the floor. No pass looks at them. The old
  registry did not list these fixtures at all, so nothing said so.
- five in own/, optional/ and generic/: a marker disagrees with the diagnostic
  about the line or the wording. Each is either a wrong marker or a wrong
  diagnostic and has to be read individually.

Everything removed is in git history.

fec/tests/own/badscop.fe

@@ -0,0 +1,12 @@
+// ERROR:9:reference
+unit badscop;
+
+fn bad(cond: bool) -> i32 {
+    let outer: i32 = 0;
+    var r: &i32 = &outer;
+    if cond {
+        let inner: i32 = 3;
+        r = &inner;
+    }
+    return r.^;
+}