Add central memory-class ladder (MemClass.h) with fail-safe-small defaults (#10901)

* Right-size nRF52 heap tiers after 2.8.0 heap-exhaustion field reports

Field reports on 2.8.0 show nRF52840 devices at 99% heap (114/115 KB)
within minutes of boot; operator new asserts on OOM, so these devices
are one allocation from a reboot. The 2.8.0 cache sizing ladders gave
nRF52 the largest non-PSRAM tiers on the assumption that a BLE-only
part has a roomy heap - the arena is actually ~125 KB shared with the
FreeRTOS task stacks.

Per-target retiers (nRF52840 unless noted):
- Traffic Management cache 1000 -> 250 entries (10 KB -> 2.5 KB); the
  unclassified fallthrough drops 1000 -> 400 to match the classic-ESP32
  tier (also affects RP2040/RP2350)
- Warm node store 200 -> 100 entries (8 KB -> 4 KB); the non-XXAA
  fallthrough drops 320 -> 100 so an unclassified RAM-constrained part
  can't boot-allocate 12.8 KB
- MESSAGE_HISTORY_LIMIT 20 -> 10 (text pool 4.4 KB -> 2.2 KB), the tier
  classic ESP32 already ships
- MAX_RX_TOPHONE 32 -> 16, shrinking the static packet pool 70 -> 54
  slots (~6.6 KB of .bss returned to the heap arena)
- PacketHistory hash index off arch-wide (1 KB); O(n) over 240 records
  is negligible at LoRa packet rates
- OLEDDISPLAY_REDUCE_MEMORY arch-wide (~1 KB OLED back buffer); the five
  TFT variants -U it because TFTDisplay.cpp needs buffer_back for
  dirty-window diffing
- Drop the stale "for testing" 1024-entry TMM override on T1000-E

Measured on rak4631: heap arena grows 124,572 -> 131,180 B and boot
allocations drop ~15.7 KB, roughly +22 KB free heap on the field-report
device class.

Migration: the nRF52840 warm flash ring replays through place() (LRU),
so the newest 100 identities survive the shrink; the file backend
rejects oversized snapshots cleanly (new test covers this). Native
suites pass (536/536 Docker, 13/13 native-macos warm store); rak4631,
heltec-mesh-node-t114 (TFT) and tracker-t1000-e build green.

* Add central memory-class ladder (MemClass.h) with fail-safe-small defaults

The 2.8.0 nRF52840 heap exhaustion happened because each RAM-sized cache
picked its per-platform tier from its own chip #ifdef ladder, and every
ladder's fallthrough default was its largest non-PSRAM tier - nRF52 was
never named, so it silently got 1000-entry caches on a ~115 KB arena.

This introduces src/memory/MemClass.h: a single MESHTASTIC_MEM_CLASS
(TINY / SMALL / MEDIUM / LARGE) ranked by usable app heap after platform
overheads, with the deliberate property that an unclassified chip lands
in SMALL - a new target boots with small caches until someone opts it up
in one visible place.

The TMM cache, warm store, MAX_RX_TOPHONE and MAX_SATELLITE_NODES
ladders in mesh-pb-constants.h now key off the class; branches pinned by
something other than RAM stay explicit and say why (nRF52840's SoftDevice
arena, RP2040's warm.dat watchdog bound). MAX_NUM_NODES intentionally
stays separate - it is flash-shaped (nodes.proto vs LittleFS), not
heap-shaped.

A per-class MESHTASTIC_BOOT_CACHE_BUDGET static_assert now covers the
three big boot-allocated caches, so the next cache-adding PR that would
blow a small platform's budget fails to compile instead of exhausting
heap in the field.

No values change for any existing target: rak4631, tbeam, rak11310 and
wio-e5 build byte-identical before/after; all ladders remain
#ifndef-guarded so variant overrides keep working.

* Address review: fix RP2350 class-table doc, add PacketRecord static_assert

- MemClass.h's class table claimed RP2350 was MEDIUM while the mapping
  ladder classifies it SMALL (with RP2040) - the table now matches the
  ladder, with a note that RP2350 is a MEDIUM candidate whenever someone
  wants to tune it up (kept SMALL here so this header stays a behavioral
  no-op).
- The boot-cache budget comment referenced a static_assert pinning
  PacketHistory::PacketRecord at 20 B that did not exist (only a layout
  comment). Add the real static_assert so the budget math in
  mesh-pb-constants.h fails to compile if the record layout changes.

Also merges develop (the base #10898 landed there as a squash, which is
what made this stacked branch conflict); develop's mesh-pb-constants.h
is byte-identical to this branch's base, so the resolution keeps the
MemClass ladder unchanged. rak4631 and wio-e5 build green;
test_packet_history 47/47.

* Address review: share PACKETHISTORY_MAX, trim policy comments

- Hoist PACKETHISTORY_MAX from PacketHistory.cpp into
  mesh-pb-constants.h (next to the MAX_NUM_NODES it derives from) so the
  constructor clamp and the boot-cache budget static_assert use one
  definition instead of hand-mirrored arithmetic that could drift. The
  expression stays valid where MAX_NUM_NODES resolves at runtime
  (ESP32-S3, portduino); the pointless 2.0 double math becomes integer.
- Trim the MemClass.h header (36 -> 16 comment lines) and the budget /
  sizing-policy comments per the repo comment-length guideline, keeping
  the class table, the fail-safe-small rule, the override mechanism, and
  the include-order constraint.

rak4631 (compile-time MAX_NUM_NODES) and heltec-v3 (runtime) build
green; test_packet_history 47/47.

src/mesh/PacketHistory.cpp

@@ -9,10 +9,6 @@
 #endif
 #include "Throttle.h"
 
-#define PACKETHISTORY_MAX                                                                                                        \
-    max((uint32_t)(MAX_NUM_NODES * 2.0),                                                                                         \
-        (uint32_t)100) // x2..3  Should suffice. Empirical setup. 16B per record malloc'ed, but no less than 100
-
 #define RECENT_WARN_AGE (10 * 60 * 1000L) // Warn if the packet that gets removed was more recent than 10 min
 
 #define VERBOSE_PACKET_HISTORY 0     // Set to 1 for verbose logging, 2 for heavy debugging