perf(zccache) Phase 4 stage 2: embedded mandatory — delete FBUILD_ZCCACHE_EMBEDDED + wrapper fallback #800

Follow-up to #789 (closed by #798).

Phase 4 stage 1 (#793) made the embedded ZccacheService the
default but kept the wrapper-mode code path alive as a safety net,
selectable via FBUILD_ZCCACHE_EMBEDDED=0. This issue is the
stage 2 cleanup: embedded is now MANDATORY, the opt-out is
deleted, every wrapper-mode escape hatch is removed.

Goal

After this PR:

• FBUILD_ZCCACHE_EMBEDDED env var does nothing — both =0 and
  =1 are ignored. (Eventually a tracing::warn! if seen, then
  removed.)
• CompileBackend::Wrapped variant gone. CompileBackend becomes
  a unit struct or a typedef for the embedded service handle.
• --no-default-features no longer drops zccache — the
  embedded Cargo feature is gone (it's just "compiled in,
  always").
• The managed zccache.exe / zccache-daemon.exe / zccache-fp.exe
  binary download path (managed_zccache.rs) is deleted.
• ~/.fbuild/<mode>/bin/zccache-* is no longer a directory fbuild
  creates.

Files to delete

• crates/fbuild-build/src/managed_zccache.rs (whole file).
• The compile_source wrapper-mode arms in
  crates/fbuild-build/src/compiler.rs (~lines 631–681 — the
  if cfg!(windows) { … wrap_args … } else { … } block + the
  run_command + retry-on-stale-daemon dance).
• wrap_args, find_zccache, discover_env_zccache,
  find_zccache_in_venv, ensure_running, stop,
  output_has_stale_daemon_error, and the ZCCACHE_PATH cache in
  crates/fbuild-build/src/zccache.rs.
• check_fingerprint / mark_fingerprint_success lose their
  wrapper-mode fallback bodies (the embedded path becomes the only
  path).
• FBUILD_NO_ZCCACHE and FBUILD_ZCCACHE_BIN env-var handling
  everywhere.
• MANAGED_ZCCACHE_VERSION references in pyproject.toml, in
  CI release matrix, and in .github/scripts/.
• The compiler_cache: Option<&Path> parameter on
  compile_source, Compiler::compile_one, and every per-platform
  compiler that threads it (avr, esp32, esp8266, rp2040, stm32,
  teensy, nrf52, nxplpc, sam, silabs, apollo3, ch32v, renesas,
  generic_arm).
• The wrapper-mode path-normalization helpers in
  crates/fbuild-build/src/zccache.rs (compile_cwd_from_output,
  path_arg_for_compile_cwd, normalize_flags_for_compile_cwd,
  strip_unc_prefix, flag_takes_path_argument,
  split_joined_path_flag) MAY be retained if the embedded
  service needs them at the call boundary. Decide per-helper.

Files to update

• crates/fbuild-build/Cargo.toml — delete the embedded feature
  flag; promote zccache = { git = …, optional = true } to an
  unconditional dep.
• crates/fbuild-daemon/Cargo.toml — drop the embedded feature
  propagation.
• crates/fbuild-build/src/compile_backend.rs — collapse
  CompileBackend to a thin wrapper around
  Arc<FbuildZccacheService>. Rename from_env to start since
  there's no env-var branching left. Delete EmbeddedServiceError:: Start fallback warnings — failure becomes a hard error (the
  daemon refuses to start if zccache can't).
• crates/fbuild-build/src/zccache_embedded.rs — keep, but rename
  FbuildZccacheService::start to clarify it's the only entry
  point. Inline the compile_blocking call site into
  compile_source since there's no longer a wrapper alternative
  to switch on.
• crates/fbuild-daemon/src/main.rs — install_global becomes
  unconditional; remove the fallback log lines.
• crates/fbuild-daemon/src/context.rs — compile_backend field
  becomes Arc<FbuildZccacheService> instead of an enum.
• crates/fbuild-build/README.md — replace the "Two backends" section
  with a "How zccache is wired" section. No more opt-out section,
  no more --no-default-features advice.
• CLAUDE.md — drop every mention of MANAGED_ZCCACHE_VERSION
  and the lockstep-bump workflow.
• docs/RELEASING.md — same.

Issues to close in the same PR

• FastLED/fbuild#32
  — the rationale for the intentionally-detached zccache start
  spawn (so the wrapper daemon could outlive fbuild-daemon) is
  moot once ensure_running is gone. The cache is now part of
  the fbuild process; it dies with the fbuild process. Add
  Closes #32 to the PR body.

Acceptance

• cargo check --workspace --all-targets clean. No
  --features fbuild-build/embedded invocation needed
  anywhere (the feature is gone).
• cargo clippy --workspace --all-targets -- -D warnings clean.
• uv run test --full green on Linux, macOS, Windows.
• grep -rn 'FBUILD_ZCCACHE_EMBEDDED\|FBUILD_NO_ZCCACHE\|FBUILD_ZCCACHE_BIN\|MANAGED_ZCCACHE_VERSION\|managed_zccache\|wrap_args\|ensure_running' crates/ docs/ CLAUDE.md
  returns no hits (except possibly in a release note / CHANGELOG).
• ~/.fbuild/<mode>/bin/zccache-* is NOT created on a clean
  run against a fresh ~/.fbuild/.
• Process snapshot during fbuild build tests/platform/uno
  shows zero zccache.exe / zccache-daemon.exe /
  zccache-fp.exe children. No wrapper binary spawn anywhere.
• crates/fbuild-build/Cargo.toml has no embedded feature
  entry. zccache dep is unconditional.
• Daemon teardown hardening: process containment + socket lingering + console events #32 closed in the same PR.

Risks / sequencing

• One release cycle of soak time on the stage-1 default flip
  (perf(zccache) Phase 4: flip embedded to default, retire managed wrapper #793) before this PR. Stage 1 shipped in feat(zccache): omnibus migration — embedded backend as default (#789 Phases 1-4) #798. Land this
  cleanup only after a release where stage-1's default has been
  exercised in the wild without regressions. The PR is irreversible
  in practice (the wrapper code is gone, not gated off), so the
  validation gate matters.
• Cross-platform CI must light up on the stage-2 PR. The
  embedded service's cache directory creation, file locking, and
  path-normalization behavior all need to work on Linux + macOS +
  Windows without the wrapper-mode safety net.
• Any consumer of find_zccache outside fbuild-build itself —
  none today AFAIK, but worth a final grep before deleting.