perf(zccache) Phase 1: embedded backend prep behind FBUILD_ZCCACHE_EMBEDDED opt-in

[zackees]

Sub-issue of #789 (zccache embedded migration).

Scope

Land the opt-in scaffolding. Default behavior unchanged — wrapper path
stays the default, embedded mode reached only via FBUILD_ZCCACHE_EMBEDDED=1.

Deliverables

• Optional embedded Cargo feature on fbuild-build pulling in the
  zccache library as a git-pinned dep (no submodule yet — see perf(zccache): migrate from managed wrapper binary to embedded ZccacheService in fbuild-daemon #789
  "vendoring" discussion).
• CompileBackend::{Wrapped, Embedded} enum held on a daemon state struct
  in fbuild-daemon. Resolved once at daemon start from
  FBUILD_ZCCACHE_EMBEDDED.
• New crates/fbuild-daemon/src/zccache_embedded.rs wrapping
  ZccacheService with fbuild's identity defaults + ~/.fbuild/<mode>/cache/
  root. Construct the service at daemon startup when embedded mode is on.
• Bump MANAGED_ZCCACHE_VERSION to match whatever revision the embedded
  library dep pins, so wrapped + embedded share an artifact format.
• fbuild-build continues to call into the wrapper today; no per-compile
  routing change yet (that's Phase 2).
• docs/ note documenting the env var.

Out of scope (other phases)

• Per-compile dispatch through the embedded service → Phase 2 (#TBD)
• Embedded fingerprint API replacing zccache fp shellouts → Phase 3 (#TBD)
• Deleting managed_zccache.rs / wrapper code path → Phase 4 (#TBD)

Acceptance

• soldr cargo build --workspace clean (default features).
• soldr cargo build --workspace --features fbuild-build/embedded clean.
• soldr cargo clippy --workspace --all-targets -- -D warnings clean
  with and without the feature.
• Default uv run test green — no behavior change without the flag.
• Smoke test: starting the daemon with FBUILD_ZCCACHE_EMBEDDED=1
  constructs the embedded service without panicking, and the daemon
  logs which backend is active.