fix(emit-lib): weak aether_lib_meta so N lib TUs link with no dup flags

[paul-hammant]

Closes #1590.

Every --emit=lib TU carries the aether_lib_meta() reflection catalog
(issue #403) with external linkage, so the 0.539 multi-TU link model —
one --emit=lib C per module, one final link, no
-Wl,--allow-multiple-definition — died with N-1
multiple definition of `aether_lib_meta' errors as soon as an
orchestrator linked two lib TUs (observed on avn: aether 0.539.0 + aeb
009c830, every ≥2-module binary). The existing multi_tu_import_link
regression used plain emission, which carries no catalog, so it passed
right through the hole.

Fix (option 1 from the issue)

The catalog entry point is now emitted weak:

#if defined(__GNUC__) || defined(__clang__)
#  define AETHER_LIB_META_WEAK __attribute__((weak))
...
AETHER_LIB_META_WEAK const struct _AetherLibMeta* aether_lib_meta(void) { ... }

• GCC/Clang/MinGW honour the attribute across ELF, Mach-O, and COFF, so
  the no-escape-hatch link works on GNU ld and ld64 — the whole
  point of the 0.539 model.
• A lone .so is byte-for-byte the same contract: one definition, dlsym
  unchanged — verified by rebuilding a single-module .so and running
  ae lib-info against it, plus the lib_meta / lib_meta_closures
  suites.
• In a deliberate multi-TU link the first TU's catalog wins; the doc now
  says to treat aether_lib_meta as meaningful only on single-module
  artifacts. (Per-TU catalog symbols — option 2 — remain open as a
  follow-up if aeb ever wants per-module introspection in one artifact.)
• Non-GNU compilers fall back to the old strong emission (single-TU
  only), same as before.

Test

multi_tu_import_link grows a phase 2 closing the coverage gap the
issue called out: the same four-module program with its three modules
emitted --emit=lib (three catalogs in one link), compiled and linked
with no duplicate-symbol flags of any kind, output pinned, plus an nm
belt that aether_lib_meta is W, not T. Before the codegen change
this phase fails with exactly the avn-reported duplicate-symbol error.

All emit-lib-adjacent suites pass locally: lib_meta, lib_meta_closures,
emit_lib_const, emit_lib_deadline, emit_lib_net, emit_csrc, emit_obj,
emit_link_requirements, multi_tu_import_link.

Downstream: once released, aeb can drop the
link_flag("-Wl,--allow-multiple-definition") workaround from its
multi-TU targets and macOS stays viable for the --emit=lib
orchestrator shape.

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