docs: PR #131 review — lifecycle ordering contract (item 9)

Documents the ImplantApp lifecycle ordering and the nav-stack state
guarantee at each hook:

- Class-level docstring on implant_app.gd describes on_install,
  on_open, on_close, and on_insert_deactivated: when each fires,
  what nav state subclasses can rely on, and what is safe to do
  (construct + register_screen in on_install; data refresh + read
  nav.current() in on_open; pause timers in on_close; no close_app
  manual call in on_insert_deactivated — call super or replicate
  the guard).

- Arch doc gains a "Lifecycle hooks" subsection under ImplantApp
  base class with a four-row contract table plus explanatory notes
  on two load-bearing invariants: why on_install sees an empty
  stack (bottom-up _ready order, no open signal yet); why on_close
  must not push/pop (would destroy preserved position on reopen).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/ui/implant/implant_app.gd

@@ -3,7 +3,34 @@ extends Control
 ## Base class for all implant apps. Absorbs HudGroups boilerplate; subclasses
 ## override lifecycle hooks only (#844, D-191).
 ##
-## Subclass _ready() pattern:
+## === Lifecycle ordering ===
+##
+## on_install() — called once from _ready(), after nav is created but BEFORE any
+##   HudGroups open event fires. The nav stack is EMPTY at this point. Use this
+##   hook to: construct screens and call register_screen(id, screen), set
+##   nav.set_default("..."), wire intra-screen signals. Do NOT rely on
+##   current_screen_id() here — no screen has been pushed yet.
+##
+## on_open(mode) — called every time HudGroups activates this app (FULLSCREEN or
+##   INSERT). By the time on_open fires, the base has ensured the nav stack is
+##   non-empty: if preserves_state=false, nav.reset_to_default() was called; if
+##   preserves_state=true and the stack was empty, nav.push_default() was called.
+##   nav.current() returns the visible screen id. Safe to read navigation state
+##   and trigger data refreshes here.
+##
+## on_close() — called every time HudGroups deactivates this app (GAMEPLAY mode
+##   or another app taking focus). Nav stack state is preserved here — do not
+##   push or pop screens in on_close. Use this hook for: pausing timers, stopping
+##   animations, unsubscribing from high-frequency feeds. The stack survives
+##   intact for the next on_open (if preserves_state=true).
+##
+## on_insert_deactivated() — called by SnapshotConsumers when the server drops
+##   insert state. The base implementation closes the app only if it is currently
+##   active in INSERT mode. FULLSCREEN apps inherit a no-op; override to add
+##   custom handling (e.g. save draft, emit warning). Do not call close_app()
+##   manually — call super() or replicate the guard condition.
+##
+## === Subclass _ready() pattern ===
 ##   func _ready() -> void:
 ##       manifest = load("res://ui/implant/apps/my_app/app.tres")
 ##       super._ready()