docs: replace invented memory figures with measured ones, ship the tuning knobs

The README claimed "~15 MB RSS" and, after the log-loading work, "~10 MB idle /
~19 MB with a 29 MB audit log". Those were TEST-INSTANCE numbers: one mock source
and one adapter. The real production config on this host (16 sources, 13
adapters, 59 models) sits at ~37-42 MB, and sat at ~105 MB before this series.
Quoting the single-source figure as the headline was misleading.

Both READMEs now state that memory scales with the number of configured sources
rather than with uptime, give a three-row measurement table (1 source / 1 source
with a 29 MB audit history / the 16-source production instance), and break the
production RSS down per region (Go heap, thread stacks + LuaJIT, mapped binary,
Go reservations, shared libs) so an operator can tell which part their own
deployment will grow.

Two runtime knobs are documented and now shipped by default in the desktop
build's core spawn (cmd/gui/main.js, overridable by exporting either variable):

  * MALLOC_ARENA_MAX=2 — LuaJIT allocates through cgo into glibc malloc, and
    glibc keeps up to 8*nproc per-thread arenas of ~1 MB that are never returned.
    Measured 8-15 arenas (7-12 MB) -> 0.
  * GOGC=50 — halves the Go heap target. Documented explicitly as useless ALONE
    (measured 20.3 -> 21.5 MB, i.e. worse, because the saved heap is eaten by
    more glibc arenas); only the pair cuts settled RSS, by ~19%.

Also corrects the binary size (8-12 MB, ~8 MB after the deploy script's -s -w)
and adds the elastic-pool / on-demand-log / self-healing-cooldown bullets that
README.md already had to README_EN.md.

cmd/gui/main.js

@@ -198,6 +198,17 @@ function startCore() {
     cwd: PROFILE_DIR,
     env: Object.assign({}, process.env, {
       LLMS_PROXY_GUI: "1",
+      // Memory tuning, measured on a 16-source deployment (see README 内存占用):
+      //   MALLOC_ARENA_MAX caps glibc per-thread malloc arenas. LuaJIT allocates
+      //     through cgo -> glibc malloc, and glibc defaults to 8*nproc arenas, so
+      //     every OS thread touching malloc reserves its own ~1 MB arena that is
+      //     never returned to the OS.
+      //   GOGC halves the Go heap growth target. On its own it does not help (the
+      //     saved heap is immediately eaten by extra glibc arenas); only the pair
+      //     lowers settled RSS. The core is I/O bound, so the extra GC is free.
+      // Both are overridable: a user who exports them keeps their own values.
+      GOGC: process.env.GOGC || "50",
+      MALLOC_ARENA_MAX: process.env.MALLOC_ARENA_MAX || "2",
     }),
     stdio: ["ignore", "pipe", "pipe"],
   });