Opt-in seeding of trybuild target from parent workspace

[TarunvirBains]

Different shape from the closed #329 (which shared the parent's target dir and ran into build-lock contention).

A new opt-in env var TRYBUILD_INHERIT_CACHE, gated behind a new optional inherit-cache cargo feature. When the feature is enabled at build time AND the env var is set at runtime, trybuild seeds its own isolated target/tests/trybuild/<crate>/debug/ from the parent workspace's target/debug/{deps,.fingerprint,incremental,build}/ before spawning cargo. The trybuild target dir override stays — preserving lock isolation from the parent.

Seeding uses reflink-copy (CoW where the filesystem supports it, plain copy fallback elsewhere — never hardlink, to avoid corrupting the parent on cargo's truncating writes). Mtime-based staleness skips the seed if the parent hasn't changed since the last run.

When the feature is off (default) or the env var unset, behavior is bit-identical to today.

Surfacing the approach here first since it's a non-trivial implementation; PR will follow.

[TarunvirBains]

Holding on this for now — initial measurement against one consumer's full trybuild surface showed the dep-cache reuse doesn't meaningfully move wall-clock (per-fixture binary compile + cargo metadata cost dominate, not the dep recompile slice). Investigating other angles; will revisit if measurements support pursuing this further.