refactor(engine): bundle apply_plan args into ApplyContext struct (#54)

apply_plan reached 9 positional parameters in M3 (5 adapters + plan
+ snapper config + 2 paths) and was carrying clippy::too_many_arguments
allows on both itself and record_apply_failure. Per the M3 retrospective
"Actions for M4" item 2, fold the system-side wiring into a borrow-only
struct so future apply-time additions (e.g. ADR-0012 nix bootstrap
preflight wiring) extend the struct rather than the call surface.

- New `ApplyContext<'a>` struct in pearlite-engine::apply with eight
  borrowed fields (5 trait-object adapters, snapper_config, state_path,
  failures_dir). Re-exported from pearlite_engine.
- `apply_plan(&self, plan, &ApplyContext)` — drops to 3 args.
- `record_apply_failure` likewise takes the context, dropping from 9
  to 6 args and removing its too_many_arguments allow.
- CLI dispatch.rs constructs the context from its existing per-call
  values; no behavior change.
- Test sites use a `make_ctx` helper for the standard mock set; the
  three injected-failure tests (FailingHm, FailingSnapper,
  FailingRestart) construct ApplyContext literals inline.

Tests: 303 passing (no change). Clippy clean. fmt clean. audit clean.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: UnbreakableMJ

crates/pearlite-cli/src/dispatch.rs

@@ -257,17 +257,17 @@ fn dispatch_apply(
             ),
         };
     }
-    match ctx.engine.apply_plan(
-        &plan,
-        ctx.pacman.as_ref(),
-        ctx.cargo.as_ref(),
-        ctx.systemd.as_ref(),
-        ctx.snapper.as_ref(),
-        ctx.home_manager.as_ref(),
-        opts.snapper_config,
-        &ctx.state_path,
-        &failures_dir,
-    ) {
+    let apply_ctx = pearlite_engine::ApplyContext {
+        pacman: ctx.pacman.as_ref(),
+        cargo: ctx.cargo.as_ref(),
+        systemd: ctx.systemd.as_ref(),
+        snapper: ctx.snapper.as_ref(),
+        home_manager: ctx.home_manager.as_ref(),
+        snapper_config: opts.snapper_config,
+        state_path: &ctx.state_path,
+        failures_dir: &failures_dir,
+    };
+    match ctx.engine.apply_plan(&plan, &apply_ctx) {
         Ok(outcome) => Envelope::success(metadata_at(Some(host)), apply_outcome_view(&outcome)),
         Err(e) => Envelope::failure(
             metadata_at(Some(host)),