fix: support platforms without fspy by gating the fspy crates out

[mfechner]

The fspy file-access tracking family (fspy, fspy_nostd, fspy_shared, fspy_shm, ...) only builds on Windows, macOS, and Linux, so any other platform could not build Vite+ at all: its always-on dependency chain (vt -> vt_server -> vt_client -> vt_ipc_shared -> fspy_ipc_str) pulled in fspy_nostd, which has no backend outside those three OSes.

Instead of porting fspy_nostd or adding no-op backends and per-item cfgs, gate the fspy crates at the crate level and keep the IPC layer portable:

• fspy_ipc_str no longer depends on fspy_nostd/fspy_nostd_alloc. The two C-string interop methods (from_os_c_str, to_os_c_string_in) move into fspy_shared as free functions built on fspy_shared's own types, so IpcStr stays a cross-platform wire type while fspy_nostd leaves the always-on graph entirely.
• vt's fspy-only execution code (FspyTracking, the TrackingIncomplete reason, is_ignored, the tracking_incomplete summary field) is now excluded via the existing cfg(fspy) when tracking is not compiled in.
• On such platforms a task requesting automatic input/output inference reports that tracking is unsupported and must configure input and output manually to be cached (FspyUnsupported), as intended.

Verified on FreeBSD 15: cargo check/clippy/test pass for the product graph (vt_bin, vt, vt_client, vt_server, vt_ipc_shared, fspy_ipc_str) and the vp binary builds and runs.