uninstall reverts a shared library's merged debug info even if another shimmed binary still needs it

When \`install --libraries\` (WithLibraries scope) shims a binary, \`mergeLibraryDebugInfo\`
(cmd/elfutil.go:116) merges external debug info into each dependency
**in place, at its real system path** — every other funkoverage target on the
system that links the same library picks up the merged copy too. The
pre-merge bytes are backed up under \`SAFE_BIN_DIR/libs/\`, tracked per
*installing* binary via a sidecar (\`funkutil.LibBackupPath\`).

\`uninstall\` (cmd/shim.go:179) calls \`restoreLibraryBackups\` (cmd/elfutil.go:161),
which restores every library in *that binary's own* backup sidecar back to
its pre-merge state — with no check for whether another still-installed
binary shares the same library and depends on the merge being there.

This is a real runtime break, not just cosmetic: \`Tracer.Start\`
(cmd/shim_binary/tracer.go:177) calls \`link.OpenExecutable(img)\` +
\`UprobeMulti\` fresh on **every invocation** of a shimmed binary, resolving
symbol names against whatever is currently on disk at \`img\`. So:

1. Install A and B, both with \`--libraries\`, both linking libfoo.so.
2. libfoo.so gets merged once (first install wins); both A and B's
func-lists include locally-scoped symbols only resolvable via that merge.
3. Uninstall A → libfoo.so is restored to its original, unmerged state.
4. B still runs fine at that instant, but its *next* invocation fails to
attach uprobes for those symbols — B's coverage silently degrades
without B itself ever being touched.

Already flagged as a known shortcut in the code:

// ponytail: no cross-target reference counting — if two installed targets
// share a library, uninstalling one restores it for both. Add a refcount
// sidecar under SAFE_BIN_DIR/libs/ if that becomes a real problem.
(cmd/elfutil.go:109-111)

Fix direction already sketched there: a refcount (or "which installed
targets reference this merged library" set) sidecar under
\`SAFE_BIN_DIR/libs/\`, consulted by \`restoreLibraryBackups\` so a shared
library is only reverted once no remaining installed target still needs it.

Start with mergeLibraryDebugInfo and restoreLibraryBackups in cmd/elfutil.go, then follow uninstall in cmd/shim.go and library resolution in cmd/shim_binary/tracer.go. Inspect the existing SAFE_BIN_DIR/libs/ backup sidecars and the ponytail comment for the intended reference tracking. Done means uninstalling one target does not restore a shared library while another installed target still needs its merged debug info, and the final target restores the original bytes.

Written by the indexing model from the issue text.