fix: refetch an installed binary that cannot run on this host; plugin 0.9.1

[emo-eth]

fetch-herdr-annotate.sh and fetch-plannotator-tui.sh skip the download when bin/<name>.exe is executable and its version stamp matches the pin. A plugin directory synced from another machine passes that check with the other machine's binary, so a macOS binary stays in place on Linux.

The shortcut now also requires bin/<name>.exe --version to succeed. A binary that cannot run on this host falls through to the normal download and SHA256SUMS check.

• Fresh installs (herdr plugin install / update), Lite, Windows Full, stage-local.sh, and the HERDR_ANNOTATE_BIN / PLANNOTATOR_TUI_BIN overrides behave as before.
• The PowerShell fetchers are unchanged.
• The tests put a binary for the other OS under a matching stamp and serve the release offline through a curl stub. They fail on main and pass here. The herdr-annotate test also checks that a checksum mismatch on this path is still fatal.
• Plugin 0.9.1 in all four manifests.

refs #54

[backnotprop]

Thanks @emo-eth, and thanks for splitting this out from #54. I pushed a trimmed version onto your branch so we can merge it now.

What I kept: the core of your fix. Before the bash fetchers say "already installed", they now run bin/<name>.exe --version. If the binary can't run on this machine (for example, a synced Mac binary on Linux), the fetcher does the normal verified download. I also added tests that put a binary for the other OS in bin/ under a matching stamp. These tests fail on main and pass here.

What I trimmed, and why:

• The cargo fallback build. Plugin installs shouldn't compile anything or need a Rust toolchain. A fallback also turned a checksum mismatch into a silent rebuild, and a checksum failure has to stop the install.
• The target stamp and the node/bun arch detection. The --version check already catches the synced-binary case. The node mapping could also pick x86_64 on an unknown Linux arch.
• The Git Bash/MINGW branches and the Windows asset names in the .sh scripts. Herdr on Windows runs the PowerShell fetchers, so those paths never run (same reason as Fix plannotator-tui fetch for native Windows (Git Bash/MSYS/Cygwin) #33).
• The PowerShell changes. The new check there didn't run the binary. Doing that properly needs real Windows test executables, so I left the .ps1 fetchers as they are for now.

herdr plugin install and update always start in a fresh directory, so normal installs work exactly as before. This only affects re-running a fetcher over an existing bin/, which is your synced setup. Plugin is now 0.9.1.