fix(init): complete home preparation and keep it retryable

[ss-o]

.zi-prepare-home recorded the home as ready before doing any work, and created the services directory only as a side effect of creating the snippets directory.

The defect

.zi-prepare-home() {
  [[ -n ${ZI[HOME_READY]} ]] && return
  ZI[HOME_READY]=1          # before anything is created
  ...
  if [[ ! -d ${ZI[SNIPPETS_DIR]} ]]; then
    ...
    command mkdir -p "${ZI[SERVICES_DIR]}"    # only reached when snippets/ is absent
  fi
}

A tree with snippets/ present and services/ missing, which a migration or a manual repair can leave behind, completes without creating the services directory and is still recorded as ready:

PROBE before: snippets=1 services=0
PROBE prepare rc=0
PROBE after:  services=0
PROBE HOME_READY=1

Service startup then places locks and FIFOs below a directory that does not exist, and ZI[HOME_READY]=1 stops any later call from repairing the layout.

The fix

Three changes, matching the acceptance criteria:

• the services directory gets its own condition, independent of snippets
• the required set is validated before readiness is recorded, naming the first missing directory
• ZI[HOME_READY] is set only after that validation passes, so a failure stays retryable

PROBE partial-layout  rc=0  services=1
zi: home preparation incomplete, missing: .../zi/services
PROBE unwritable      HOME_READY=unset
PROBE retry           rc=0  services=1  HOME_READY=1

What is deliberately not required

$ZPFX/{bin,lib,share} and ZI[MAN_DIR] stay best-effort. They are user-facing install prefixes rather than Zi's own state, and the manual tree has always tolerated a creation failure through command mkdir 2>/dev/null -p. Promoting them to required would turn a read-only prefix into a startup failure for shells that work today, which is a bigger behaviour change than this issue asks for. The comment in the source says so.

Test

tests/home-preparation.zsh covers the partial layout, a required directory that cannot be created, the retry once the cause is removed, and the no-op on an already-ready home. It skips itself under root, where the permission case cannot hold, and restores permissions in its trap so cleanup works.

Against the previous function:

partial layout: the services directory was not created
not ok - home preparation is not complete, propagating or retryable    exit=1

Registered in zsh-n.yml, both path filters and a job. Full suite 25/25.

Closes #448