perf(zccache) Phase 4: flip embedded to default, retire managed wrapper

[zackees]

Sub-issue of #789. Depends on Phase 3.

Scope

Once Phase 2 + 3 have shipped on main for at least one release without
regressions, flip embedded to the default and remove the wrapper code
path entirely.

Deliverables

• CompileBackend::Embedded becomes the default. FBUILD_ZCCACHE_EMBEDDED
  env var deleted (was only a Phase 1 opt-in).
• Delete crates/fbuild-build/src/managed_zccache.rs and every reference
  to it.
• Delete the zccache start / ensure_running path and the wrapper-arg
  helpers (wrap_args, find_zccache, discover_env_zccache,
  find_zccache_in_venv) from crates/fbuild-build/src/zccache.rs.
• Re-evaluate the path-normalization helpers (compile_cwd_from_output,
  normalize_flags_for_compile_cwd, path_arg_for_compile_cwd,
  strip_unc_prefix): keep what the embedded service needs, delete the
  rest. zccache.rs should shrink substantially.
• Delete FBUILD_NO_ZCCACHE and FBUILD_ZCCACHE_BIN env vars — they
  were wrapper-mode escape hatches.
• Update CLAUDE.md, docs/, and any README that references the managed
  binary download or the env vars.
• Strip MANAGED_ZCCACHE_VERSION from pyproject.toml and from the
  release/CI matrix.
• Close Daemon teardown hardening: process containment + socket lingering + console events #32 once ensure_running is gone — its rationale
  is now moot.

Acceptance

• ~/.fbuild/<mode>/bin/zccache-* is never created on a clean run.
• grep -rn zccache_wrap\|managed_zccache\|FBUILD_ZCCACHE_BIN\|FBUILD_NO_ZCCACHE
  returns no hits in crates/, docs/, CLAUDE.md.
• uv run test --full green on Linux, macOS, Windows.
• CI release matrix no longer references MANAGED_ZCCACHE_VERSION.
• perf(zccache): migrate from managed wrapper binary to embedded ZccacheService in fbuild-daemon #789 can be closed.

[zackees]

Fixed in #798 (merged). Phase 4 stage 1: embedded is now in default features for both fbuild-build and fbuild-daemon, and FBUILD_ZCCACHE_EMBEDDED is an opt-out (=0) instead of an opt-in (=1). Wrapper-mode code retained as the safety-net fallback; code deletion + env-var removal tracked as a Phase 4 stage 2 follow-up after one release cycle of validation.