Validate save imports by main-data checksum, not raw file size

The SAVE FILES card only accepted saves of exactly 32768 bytes and
refused anything else. importToSlot now classifies a non-32768 file by
the integrity of its main-data checksum instead:

- Oversize + valid checksum -> an emulator RTC footer, so the launcher
  asks for confirmation, then truncates to 32768 on force.
- Oversize + invalid checksum -> rejected.
- Undersize + valid checksum -> imports zero-padded; otherwise refused.

Adds the "Oversized save file" confirm modal, a new vendor-oracle test
built by gen1lib (PKHeX-derived) run as its own Lua 5.4 tier, oversize/
truncated policy tests, and the LUA54 wiring in test.sh.

# Conflicts:
#	src/import/LauncherView.lua

src/save_convert/GenSave.lua

@@ -190,6 +190,18 @@ local function checksum(bytes, from, to)
   return bit.band(bit.bnot(sum), 0xFF)
 end
 
+-- Main-data checksum gate used before an import policy is decided.  Returns
+-- nil when the buffer is too short to even carry the stored checksum byte
+-- (offset O.mainChecksum, the last byte of wMainData), false on a mismatch,
+-- true when it matches.  Works on any length >= O.mainChecksum + 1, so a
+-- caller can classify a truncated or footer-padded file without a full
+-- decode -- the checksummed region (0x2598..0x3522) always sits entirely
+-- inside the first 0x3524 bytes of a save.
+function GenSave.mainChecksumValid(bytes)
+  if #bytes < O.mainChecksum + 1 then return nil end
+  return checksum(bytes, O.checksumStart, O.checksumEnd) == u8(bytes, O.mainChecksum)
+end
+
 -- flag_array packs LSB-first within each byte (bit 0 of byte 0 = index 0).
 -- This is pokered's runtime FlagAction convention (home/predef macros): it
 -- takes flag number N, addresses byte N/8, and builds the mask by rotating

src/save_convert/SaveConvert.lua

@@ -25,6 +25,7 @@ local GenSave = require("src.save_convert.GenSave")
 local SaveConvert = {}
 
 SaveConvert.SAVE_SIZE = GenSave.SAVE_SIZE
+SaveConvert.mainChecksumValid = GenSave.mainChecksumValid
 
 -- ------------------------------------------------------------------
 -- Crosswalk data loading (cached).  Mirrors src/core/Data.lua: prefer