[nanvix] E: Phase 4 — build lxml C extensions as .so

Phase 4 of the .a -> .so migration (see
nanvix-todo/cpython-static-to-shared-migration.md section 8).
Promotes the 2 lxml C extension modules (_lxml_etree,
_lxml_elementpath) from statically linked into python.elf to
dlopen-loaded shared objects.

Modules moved from *static* to *shared* in Modules/Setup.local
generation (.nanvix/docker.py):

- _lxml_etree (links liblxml_etree.a + libxslt.a + libexslt.a +
  libxml2.a + libz.a from the sysroot)
- _lxml_elementpath (links liblxml_elementpath.a + libxml2.a +
  libz.a)

Both modules use the same thin-shim approach the static variant
used: lxml_etree_builtin.c / lxml_elementpath_builtin.c each
register the C extension's PyInit_ entrypoint after pulling in
the Cython-generated implementation from
liblxml_etree.a / liblxml_elementpath.a in the sysroot.

The shim files are unchanged. The lxml/etree.py Python-level
shim that re-exports symbols from `_lxml_etree` continues to
work as-is, because Python's import system finds
.cpython-312.so files in lib-dynload/ exactly like static
modules — the import name `_lxml_etree` resolves either way.

Same trade-off as Phase 2 and Phase 3: each .so embeds its own
copy of the underlying libraries (libxslt, libexslt, libxml2,
libz). _lxml_etree.so is 3645 KB primarily because libxslt and
libxml2 are sizable. _lxml_elementpath.so is 175 KB because it
only needs libxml2.

This trade-off is tracked alongside the Phase 2 + Phase 3 bundled
libs in nanvix-todo/phase2-3-unbundle-bundled-libs.md as a
deferred follow-up optimization PR. Note that libz is already
shared with Phase 3's zlib.so today (each carries its own copy);
the same applies to libxml2 (only used by lxml; if libxml2 had
to be in python.elf for other consumers we'd see the same
visibility merge issue as libm, but it has no Rust
compiler_builtins shadows so we're safe).

Validation on local toolchain (phase0-llfix):

- Both .so files produced and installed under lib-dynload/.
- nm python.elf no longer shows PyInit__lxml_etree or
  PyInit__lxml_elementpath.
- python.elf size: 11.20 MB (Phase 3) -> 8.48 MB (Phase 4),
  -2.72 MB. python.elf now meets the migration plan's "<=10 MB
  stripped" success criterion from Section 10.
- Hello + all Phase 1/2/3 import probes + lxml import-and-parse
  smoke + HTTP server smoke + full regrtest 160/160 PASS in
  standalone mode.
- test_nanvix_lxml (regrtest's lxml-specific test) included in
  the 160/160 and passes.

This completes the migration plan's Phase 4 scope — only Phase 5
(optional dormant module revival for numpy / regex) remains.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Enrique Saurez

.nanvix/docker.py

@@ -293,14 +293,15 @@ def _generate_setup_local_cmd() -> str:
         f"'*static*' "
         f"'# Nanvix OS interface module (snapshot, host-mount).' "
         f"'_nanvix _nanvixmodule.c' "
-        f"'# lxml C extension modules (statically linked via pre-built archives).' "
-        f"'_lxml_etree lxml_etree_builtin.c -L{sysroot}/lib -llxml_etree -lxslt -lexslt -lxml2 -lz' "
-        f"'_lxml_elementpath lxml_elementpath_builtin.c -L{sysroot}/lib -llxml_elementpath -lxml2 -lz' "
         f"'' "
         f"'# Phase 0 of the .a -> .so migration: array as proof-of-concept shared module.' "
         f"'# See nanvix-todo/cpython-static-to-shared-migration.md section 4.' "
         f"'*shared*' "
         f"'array arraymodule.c' "
+        f"'# Phase 4: Tier-4 third-party native .a (lxml). Each module is a' "
+        f"'# thin shim that loads the pre-built per-module .a from sysroot.' "
+        f"'_lxml_etree lxml_etree_builtin.c -L{sysroot}/lib -llxml_etree -lxslt -lexslt -lxml2 -lz' "
+        f"'_lxml_elementpath lxml_elementpath_builtin.c -L{sysroot}/lib -llxml_elementpath -lxml2 -lz' "
         f"'# Phase 1A: Tier-1 data primitives (pure C, no external deps).' "
         f"'# See nanvix-todo/cpython-static-to-shared-migration.md section 5.' "
         f"'_bisect _bisectmodule.c' "