Enable the on-demand mOTA context for every classic ESP32 build

Move OTA_HEAP_CONTEXT from two hand-edited envs to a pre-script gated on
build.mcu == "esp32", so every classic ESP32 image gets it and none can drift.
S2/S3/C-series and nRF52 keep the .bss singleton: they have no equivalent
static-DRAM ceiling, so there a guaranteed-present workspace is the better
trade for what is ultimately a recovery path. It has to be a build flag rather
than a header default, because OTA_HEAP_CONTEXT must hold the same value in
every translation unit that sees OtaContext.h - OtaContext.cpp included - and a
header test on CONFIG_IDF_TARGET_ESP32 would depend on include order relative
to sdkconfig.h. The script defers to OTA_SHARED_COMPANION_QUEUE where a Full
Companion recipe already owns the storage.

Fix a gap this exposes in the roles that now use it. Only CLI entry points
acquire the context, so once it was released, a repeater with the temporary
radio profile up would no longer serve or announce its own firmware - LoRa OTA
would look enabled and silently do nothing. ota_service_temp_radio_context()
holds the workspace for exactly the temp-radio window and hands it back
outside it, which is where the saving was coming from anyway. Wired into the
repeater, room server and sensor loops; the Companion keeps its own
acquire-on-host-demand policy, since its context is borrowed from the offline
message queue and must not be taken speculatively.

Static DRAM, classic ESP32 (bytes occupied of a 124,580 region):
  Heltec_v2_repeater                   108,332 -> 70,732
  Heltec_v2_room_server                 83,716 -> 68,300
  Heltec_v2_companion_radio_ble        123,236 -> 107,772  (was failing on main)
  Tbeam_SX1262_repeater                 86,932 -> 71,468
  Tbeam_SX1262_repeater_observer_mqtt  121,132 -> 83,532   (was failing)

scripts/esp32_ota_heap_context.py

@@ -0,0 +1,56 @@
+#!/usr/bin/env python3
+"""Keep the idle mOTA workspace off classic ESP32's static DRAM.
+
+Classic ESP32 links into a dram0_0_seg of ~124 KiB: memory.ld reserves the BT
+controller's 0xdb5c bytes off the 0x2c200 window before the application gets
+any, and no runtime call gives those static bytes back. OtaContext is several
+kilobytes of that budget, held for the life of the device even though a
+repeater or room server is outside an OTA window essentially always.
+
+OTA_HEAP_CONTEXT switches OtaContext to the on-demand storage path, so it is
+allocated when an OTA operation needs it and freed once idle. Applied here
+rather than per-env so every classic ESP32 build gets it and none can drift.
+
+Scoped to build.mcu == "esp32" deliberately. S2/S3/C-series and nRF52 have no
+equivalent static-DRAM ceiling, so there the .bss singleton is the better
+trade: it guarantees the workspace is present, and OTA is a recovery path.
+
+This must be a build flag, not a header default. OTA_HEAP_CONTEXT has to hold
+the same value in every translation unit that sees OtaContext.h - including
+OtaContext.cpp, which defines the storage - and a header test on
+CONFIG_IDF_TARGET_ESP32 would depend on whether that unit had already reached
+sdkconfig.h.
+"""
+
+Import("env")  # noqa: F821 -- PlatformIO/SCons supplies Import
+
+# Both name the owner of the context storage, and OtaContext.h rejects the
+# pair. OTA_SHARED_COMPANION_QUEUE arrives via PLATFORMIO_BUILD_FLAGS from
+# build.sh's Full Companion recipes, which are nRF52/S3 today - but a future
+# classic ESP32 Full Companion must lose this default, not fail to compile.
+CONFLICTING = ("OTA_HEAP_CONTEXT", "OTA_SHARED_COMPANION_QUEUE")
+
+
+def _already_defined(env):
+    for define in env.get("CPPDEFINES", []):
+        name = define[0] if isinstance(define, (list, tuple)) else define
+        if str(name) in CONFLICTING:
+            return True
+    # PLATFORMIO_BUILD_FLAGS reaches BUILD_FLAGS as raw text, which may not be
+    # parsed into CPPDEFINES yet when this pre-script runs.
+    for flag in env.get("BUILD_FLAGS", []):
+        text = str(flag)
+        if any(("-D" + name) in text for name in CONFLICTING):
+            return True
+    return False
+
+
+def _apply(env):
+    if str(env.BoardConfig().get("build.mcu", "")).lower() != "esp32":
+        return
+    if _already_defined(env):
+        return
+    env.Append(CPPDEFINES=[("OTA_HEAP_CONTEXT", 1)])
+
+
+_apply(env)  # noqa: F821