fix(flatpak): prune pf-client-core's windows-rs git dep from the sandbox manifest — offline build broken since a69a83b5

The flatpak's offline cargo build has failed on every run since the D3D11VA push:
pf-client-core now declares the same git-pinned `windows` dependency as the Windows
client (cfg(windows)-gated, never compiled on Linux), and `cargo --offline` needs
every DECLARED dependency's source just to build the unit graph — but windows-rs is
deliberately not vendored into cargo-sources.json (flatpak-builder would full-clone
the multi-GB repo; the reason prune-windows-lock.py exists). Removing the workspace
member alone no longer covers it.

New packaging/flatpak/prune-windows-toml.py (dependency-free, like its lock sibling)
strips windows-rs git entries from a manifest in place — single- or multi-line —
and the flatpak manifest runs it on crates/pf-client-core/Cargo.toml right after the
existing clients/windows member sed. Registry deps in the same cfg(windows) table
(wasapi, sdl3) are kept; they vendor normally.

Verified in a scratch worktree with both prunes applied: the TOML stays valid and
`cargo metadata` resolves ZERO windows-rs packages and zero git-source declarations
— nothing left for --offline to fetch. End-to-end proof is the flatpak CI run on
this push.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packaging/flatpak/prune-windows-lock.py

@@ -1,12 +1,15 @@
 #!/usr/bin/env python3
 """Write a copy of a Cargo.lock with the microsoft/windows-rs git packages removed.
 
-The punktfunk workspace lockfile includes `punktfunk-client-windows`' git dependencies — the
-whole microsoft/windows-rs tree (windows-reactor + ~13 `windows-*` crates, all pinned to one
-git rev). The flatpak builds ONLY the Linux client (`-p punktfunk-client-linux`), which never
-references them, but `flatpak-cargo-generator.py` walks the whole lock and emits a `type: git`
-source for windows-rs; `flatpak-builder` then FULL-clones that multi-GB repo at "Downloading
-sources" → "No space left on device", failing the build.
+The punktfunk workspace lockfile includes the windows-rs git dependencies — the whole
+microsoft/windows-rs tree (windows-reactor + ~13 `windows-*` crates, all pinned to one git
+rev) declared by `punktfunk-client-windows` and `pf-client-core` (D3D11VA). The flatpak
+builds ONLY the Linux binaries, which never compile them, but `flatpak-cargo-generator.py`
+walks the whole lock and emits a `type: git` source for windows-rs; `flatpak-builder` then
+FULL-clones that multi-GB repo at "Downloading sources" → "No space left on device",
+failing the build. (The manifests that DECLARE the git dep are neutralized in the build
+sandbox too — see prune-windows-toml.py — since `cargo --offline` needs every declared
+dependency's source even for cfg(windows)-gated entries it never compiles.)
 
 Stripping those packages from the lock the generator sees is safe: the committed Cargo.lock is
 left untouched for the `--locked` build, and `cargo --offline` only needs vendored sources for