adopt running_process::blake3_file for dev daemon-identity isolation

[zackees]

Context

running-process now exposes a shared content-hash primitive for dev daemon-identity isolation (landed via zackees/running-process#891 / PR zackees/running-process#892):

// feature = "client"
pub fn running_process::blake3_file(path: &Path) -> std::io::Result<blake3::Hash>;

It hashes a file's bytes with blake3::Hasher::update_mmap_rayon — content-based (so same-version rebuilds get distinct identities), mmap + multi-core (page-cache-warm, ~1–3 ms for a 20 MB binary), and immune to the ASLR/IAT/.data mutation that makes the loaded image a per-run nonce.

The problem this fixes

soldr-daemon, FastLED/fbuild, and standalone zccache all obtain their daemon identity/discovery through running-process. In dev, two builds sharing one home root (e.g. ~/.soldr-dev) rendezvous on the same daemon pipe + pid file, so each sees the other as "stale-version" and displaces it on every invocation — a displace-stale war that wedges the compile daemon (NotRunning → thousands of uncached fallbacks). Full root-cause + evidence: zackees/soldr#2352.

The shims are already version-namespaced; the daemon identity is not.

Ask: adopt the primitive here

Dev builds only (official/release builds get no stamp → bare identity, single-daemon-on-upgrade prod semantics — only dev pays the cost):

stamp = "<version>-<first-16-hex of blake3_file(current_exe)>"   // e.g. 4.9.0-a1b2c3d4e5f6a7b8

Client/daemon agreement — propagate the VALUE, not the path:

• Compute the stamp once at the top level and export it down the process tree, so every child + the spawned daemon inherits the value (one hash per build, not one per short-lived wrapper — a 200-crate build would otherwise re-hash ~200×).
• Propagating the value (not the path) is also what makes it immune to the Windows self-update lock-rename dance: a running .exe is locked so updates rename it aside; the path is not a stable key but the already-computed hash value is.

Refs: zackees/soldr#2352 (design of record + evidence), zackees/soldr#2353 (interim soldr-side per-version fix to be swapped for this), zackees/running-process#891, zackees/running-process#892.

fbuild-specific

fbuild gets its daemon identity via zccache. Once zccache honors the inherited ZCCACHE_DAEMON_NAMESPACE stamp, fbuild inherits isolation for free — the work here is to ensure fbuild's dev invocations compute the stamp once at the top level (or inherit it from a parent soldr/zccache) and propagate the value down to every child and the spawned daemon, so co-located dev builds do not wage a displace-stale war on one pipe.