perf: re-enable native-comp JIT and hand GC to gcmh

early-init.el disabled JIT native compilation with (setq native-comp-deferred-compilation nil), the obsolete alias of native-comp-jit-compilation. Despite the comment, setting it nil turns JIT off entirely rather than making it synchronous. Most modules then ran interpreted for the daemon's lifetime, and the native-comp-speed/jobs settings in system-defaults.el were dead. The "Selecting deleted buffer" async race that prompted the disable was an Emacs 28/29 issue. This is 30.2. I re-enabled it with native-comp-jit-compilation t and silent async warnings.

GC was pinned at the stock 800KB: early-init restored it post-startup and the minibuffer setup/exit hooks bounced back to it. That's Emacs's bare-editor default, far too low for 184 packages, so GC pauses fired often during completion, agenda, and LSP/AI work. I replaced both hand-rolled mechanisms with gcmh, which keeps the threshold at 1GB during activity and collects on idle.

Verified a clean full launch in a throwaway daemon (JIT on, gcmh active, no backtrace) and gcmh's threshold cycle in batch.

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