projects phase 3: follow an import into the binary that implements it

Following a call to strcmp reached the PLT/extern entry and stopped there —
Hex-Rays has nothing to decompile, because the code lives in a library this
binary only references. With the other binary open in the same project we
already had everything needed to cross that gap; we just weren't indexing it.

Index each binary's imports and exports (KIND_IMPORT / KIND_EXPORT) alongside its
functions and strings. On a follow, _import_stub asks whether the target address
is one of this binary's import stubs; if so _cross_binary_impl asks the index who
exports that name, and we switch there instead of landing on the thunk.

Verified end to end on a real echo + libc project: Enter on `strrchr(a1, 47)` in
echo's pseudocode switches to libc.so.6 and lands on strrchr at 0xaf960.

Three things it turns on:

* ELF symbol versioning. The importer sees strrchr@@GLIBC_2.2.5 while the
  provider may export any of three spellings, so raw names resolve almost
  nothing. domain.link_name() cuts at the first '@'; Linkage.raw keeps what IDA
  reported, which is what the listing shows.
* Exact match, not substring — ProjectIndex.exact(), so `read` doesn't bind to
  pread/read_line/thread_start. It also answers below the 3-char trigram floor,
  and plenty of real exports are that short.
* Resolution reads the on-disk index, so a provider resolves while its worker is
  evicted. That's what the index was for.

When nothing in the project provides the symbol _follow_import declines and the
normal navigation runs: landing on the stub is still the honest answer, and a
single-binary session is unchanged. The PLT-stub PRESENTATION item stays open —
an unprovided import should say "imported, provider not in project" rather than
show a decompiler error.

server/patch_server.py gains list_linkage (idautils.Entries + enum_import_names);
a worker without it degrades to no linkage rather than failing.

tests: index join +8 (exact vs substring, short names, exclude-self, reverse
join, kind isolation, forget unresolves) and link_name +4. 36/0 index, 195/0
scenarios, 23/0 project UI, 33/0 project, 22/0 pool.

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