packaging(win): update INSTALL.md + README.md for 0.3.0

- INSTALL.md: bump the setup filename to 0.3.0; add an end-user section
  on text chat + sending photos/files (inline images, file chips,
  Save/Download, 25 MB cap, session-only); note that both ends must run
  the same version under "won't connect".
- README.md: add a Version compatibility section (installer version
  tracks Cargo; a 0.x MINOR bump is a breaking wire change so everyone
  must reinstall; 0.3.0 can't talk to 0.2.x).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packaging/windows/README.md

@@ -9,6 +9,18 @@ notification chimes, and avatar presets are all embedded in the binary
 runtime, so there are no extra DLLs to bundle. The installer payload is just the
 `.exe` plus an `.ico` for the Start-menu / desktop shortcuts.
 
+## Version compatibility
+
+The installer version tracks the crate version in `Cargo.toml` (currently
+**0.3.0**) — keep `MyAppVersion` in `peerspeak.iss` in sync when it changes.
+
+Per `VERSIONING.md`, a **MINOR** bump in `0.x` is a **breaking wire change**:
+peers on different MINOR versions can't connect (they fail fast at the
+handshake rather than misbehaving). So when you ship a new Windows build after
+a MINOR bump, **everyone on the call must reinstall** — an old Windows build
+and a newer Linux/Windows peer won't talk. (0.3.0 was the chat file-sharing +
+per-peer noise-gate release; it cannot connect to a 0.2.x peer.)
+
 ## Files
 
 | File | Tracked | Purpose |