feat(session): spec mode — the renderer stops resolving policy and stops writing settings

C2 of design/client-architecture-split.md, and the last of the session's overreach.

`--resolved-spec <path>` hands the session everything it needs already resolved —
effective settings, the host's clipboard decision, the profile's name — and in that mode it
performs ZERO store reads. It had been re-deriving all three, which meant policy was being
evaluated inside the thing that draws pixels, and that the spawner and the child could
disagree about a file either of them might have written in between. First-party spawns
(the shells, the CLI) always pass one now.

The compat path stays for hand-run `punktfunk-session --connect` and old Decky scripts —
but it calls the SAME helper, so the two modes cannot drift; it is the identical function
invoked in-process instead of by the parent. A spec that is named but unreadable fails
loudly rather than quietly falling back: a spawner that asked for exact settings must not
get store-derived ones instead.

The match-window write-back is gone too. The callback used to load-modify-save the shared
settings file from inside the renderer — one of that file's five concurrent writers, for a
value only the parent needs. It now reports `{"window":{w,h}}` on stdout and the spawner
persists it, on a real change only. A hand-run session still persists its own window,
because nobody is listening to its stdout there and the event alone would drop the value.

Verified on .21: a spec naming a profile that doesn't exist in the catalog is honoured
(proving no lookup happened), a missing spec errors instead of falling back, and the CLI's
spec file is written and cleaned up per launch.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

clients/linux/src/spawn.rs

@@ -78,6 +78,12 @@ pub fn spawn_session(
         wake: false,
         connect_timeout_secs: opts.connect_timeout_secs,
         tofu,
+        // The per-host clipboard decision, resolved here so the child doesn't look it up
+        // again — matched by address, the way every other per-host lookup matches.
+        clipboard: pf_client_core::trust::KnownHosts::load()
+            .hosts
+            .iter()
+            .any(|h| h.addr == req.addr && h.port == req.port && h.clipboard_sync),
     };
 
     let persist_paired = opts.persist_paired;
@@ -96,6 +102,8 @@ pub fn spawn_session(
             trust_rejected,
         } => error = Some((msg, trust_rejected)),
         SessionEvent::Ended(msg) => ended = Some(msg),
+        // The brain persists the window size; the shell has nothing to do with it.
+        SessionEvent::Window { .. } => {}
         SessionEvent::Exited(code) => {
             let _ = sender.send(AppMsg::SessionExited {
                 req: req.clone(),