Debugging c_import interop

c_import "libc.so.6" "/usr/include/stdio.h" runs three stages, all
compiled into the compiler (design doc: docs/projects/c_import.md):

1. Preprocess — libs/extras/c_preprocessor/: macro expansion,
   conditionals, #include resolution, predefined platform macros
   keyed off the target word size.
2. Parse — libs/extras/c_import/generated_c_parser.w, a PEG
   parser generated from tests/parser_generator/c.pg.
3. Import — libs/extras/c_import/importer.w: lowers typedefs,
   structs (C-aligned via inserted __ci_pad_* filler fields),
   enums, and extern functions (as weak dynamic symbols); exports
   integer object-like macros as constants.

Reproduce failures cheaply with ./bin/wv2 check file.w (insert x64
before check for the 64-bit target) — no binary is written.

Failure shapes

• c preprocessor: could not read <path> — bad header path (the
  c_import path itself gets no existence pre-check).
• c preprocessor: #error in <header>: <text> — a live #error
  branch, usually a conditional the predefined macros do not satisfy.
• <header>:line:col: syntax error: expected ..., found ... followed
  by c_import: header parse failed in <file>:<line> — the parser
  reports the furthest token any parse attempt reached; usually a
  declaration shape the C grammar lacks.
• A function silently missing: first-definition-wins collision (W's
  own open/read/malloc wrappers keep priority over libc's), or
  a static/inline declaration, which is declared-but-skipped.
  Compile with -v to surface note: c_import skipped '<name>': ...
  (a note, not a warning — -v --strict still passes).
• A call that faults at runtime: imported functions are weak, so
  a symbol the library does not actually export loads as a null GOT
  slot instead of failing at startup. A hand-written extern
  declaration stays strong and fails at load time instead.
• Struct layout drift inside bit-field regions: bit-field members are
  skipped, so that region's layout is coarse.

Pinning a fix: torture fixtures

Regressions go into the compile-only fixture group
c_import_torture_test, following tests/c_import_torture_fixture.w:
a tests/<name>_fixture.w + local tests/<name>_fixture.h pair whose
W body uses every imported shape, so an import regression fails the
compile. The fixture's header comment carries # expect_stderr: /
# reject_stderr: / # expect_fail directives asserted by
bin/wfixture, plus # wbuild: fixture_group=c_import_torture_test
to join the group (the generated manifest picks it up on the next run),
then ./wbuild c_import_torture_test.

Env-blocked targets and their substitutes

c_import_test, c_import_errno_test and c_import_libc_test
produce 32-bit dynamically linked binaries that need the i386 loader
(/lib/ld-linux.so.2, package libc6:i386); without it they fail at
exec with "required file not found". On such hosts assert through the
compile-only fixture targets (c_import_torture_test,
c_import_error_directive_test, c_import_missing_header_test,
c_import_verbose_note_test) or the x64 twins
(c_import_libc_64_test, x64_c_import_knr_test), which run with the
host's own x86-64 loader.