Raise avatar cap 64 to 128 KiB, pipeline checks encoded size

64 KiB rejected normal 128px PNGs. 128 KiB fits any 128x128 PNG
(raw <= 64 KiB, worst-case base64 ~95 KiB chars) while still
bounding abuse. PNG kept for transparency. Pipeline now measures
the encoded base64 against RelayConstants (was checking the wrong
variable with its own 512 KiB limit) and drops dead crop code.
Server policy, still v4. Trade noted: avatars ride the 1 Hz room
broadcast, uplink scales with cap; hash-reference follow-up later.

HANDOFF.md

@@ -64,7 +64,8 @@ kick kills sync+video, no relay memory growth.
 
 - `packages/core/lib/src/`: constants.dart (tuning: seek 300 ms,
   drift 5 s, slow 1.5 s @0.95, min_buffer 4 s, heartbeat 1 s, ping 5 s,
-  avatar 64 KiB, name 32, chat 500; dropAfter lives as RoomEngine ctor
+  avatar 128 KiB (base64 chars; any 128x128 PNG fits — raised 09-12
+  from 64 KiB, which rejected normal PNGs), name 32, chat 500; dropAfter lives as RoomEngine ctor
   default, now 30 s), errors.dart, logging.dart, config.dart,
   protocol.dart (v4 schemas), media_source.dart, sync_engine.dart
   (RoomEngine + quorum in evaluate's HELD branch; PLAYING branch
@@ -149,10 +150,9 @@ Client: 5 s room-silence ⇒ dead (clear room, warn, reconnect loop
    asks-leave, global focus-proof keys (Space/Tab/arrows/ESC/F,
    350 ms throttle, ignored while typing; Tab traversal disabled on
    the session screen 09-12 — game-style, Tab toggles the sheet), single media field
-   (parseMediaInput + tests), avatar pipeline (128 px PNG as of
-   09-12 uncommitted change, was JPEG — NOTE: server still
-   enforces 64 KiB in session.dart vs pipeline's 512 KiB check,
-   mismatch open, see commit), tap to
+   (parseMediaInput + tests), avatar pipeline (128 px PNG, keeps
+   transparency — JPEG did not; self-check measures encoded base64
+   against RelayConstants, single source of truth), tap to
    pick, no path fields), Discord-like corner profile (connect only;
   welcome is title+Host/Join; edits apply next join — no rename
   intent on the wire), dialogs for terminal states.