feat(icons): refresh per-platform icon assets from new 256x256 master - #15

Stages the Flathub-strict version of the aMule Flatpak manifest in `packaging/flathub/` — the file that will be submitted to flathub/flathub once domain attestation for the app-id is resolved. Differences from the internal `packaging/linux/flatpak/org.amule.aMule.yaml.in`:

- All `${...}` template variables resolved to concrete values (Flathub forbids envsubst).
- The amule source: `branch:` swapped for `tag: + commit:` (Flathub forbids mutable refs).
- The cryptopp source: existing tag augmented with a pinned commit.

Pinned at master tip post-#15 (icon refresh), #17 (metainfo / desktop polish), and #19 (ENABLE_BFD option). Carries -DENABLE_BFD=NO so the Flathub-built bundle inherits the libbfd-runtime-missing fix.

Companion file `packaging/flathub/README.md` documents the two-manifest layout, the submission flow against flathub/flathub:new-pr, and how to refresh the pin on each new aMule release tag.

…#21)

Reverts the icon-design change introduced in #15 while preserving #15's structural improvements (256x256 hicolor install path, metainfo screenshot URL pinning to the 3.0.0 tag).

Context in amule-org/amule-org.github.io#156: #15 was intended as a resolution refresh but ended up swapping to a noticeably different design. The team will revisit a proper identity refresh on its own track later.

- org.amule.aMule.png (128x128): restored bit-identical to the pre-#15 version.
- org.amule.aMule-256.png (256x256): regenerated via Lanczos upscale of the restored 128.
- amule.ico: rebuilt with sizes 16/32/48/128/256 from the restored 128.
- platforms/MacOSX/amule.icns: rebuilt with iconset sizes up to 256 (drops #15's 512/1024 @2x slots since we have no native source data above 256).

…atforms

The macOS-only `std::_Exit(0)` block in OnExit() was added to dodge a
wxWebSessionURLSession dtor bug under wx 3.3.2. Turns out the
Linux backend (wxWebSessionCURL, wx 3.2.6) hits the same class of
shutdown crash: its dtor calls curl_multi_cleanup, libcurl invokes the
registered socket callback (wxWebSessionCURL::SocketCallback) to drop
tracked sockets, and that callback dereferences session state the
dtor's earlier steps have already torn down. wxASSERT fires,
wxFatalSignalHandler raise(SIGABRT)s.

Verified with a debug-symbol Flatpak + gdb on a user-supplied core,
fully symbolicated:

    #19 main                                amule-gui.cpp:98
    #18 wxEntry                             wx/init.cpp:500
    #17 wxEntryCleanup                      wx/init.cpp:205
    #16 wxModule::CleanUpModules            wx/module.cpp:191
    #15 wxModule::DoCleanUpModules          wx/module.cpp:200
    #14 wxRefCounterMT::DecRef
        wxWebSession::Close                 wx/webrequest.cpp:1074
    #13 wxWebSessionCURL::~wxWebSessionCURL wx/webrequest_curl.cpp:932
    #12 wxWebSessionCURL::~wxWebSessionCURL wx/webrequest_curl.cpp:929
    #11-#6 libcurl curl_multi_cleanup
    #5  wxWebSessionCURL::SocketCallback    wx/webrequest_curl.cpp:1047
    #4  raise()
    #3  wxFatalSignalHandler                wx/unix/utilsunx.cpp:1523
    #0  abort

Same justification used for the macOS guard applies on every
platform: by this point in OnExit we have saved state, joined
threads, and flushed logs -- nothing aMule-owned remains to clean up.
_Exit bypasses atexit and static destructors uniformly, so the buggy
wx dtor never runs and the process terminates cleanly.

Remove this once the upstream wx fix lands in a release we depend on.

Reported in #18.