fix backup GPT partition entries offset to respect UEFI minimum array size

[andiradulescu]

Fixes [gpt] Partition entries CRC32 mismatch warnings seen after repairGpt.

asAlternate() computed the backup partEntriesStartLba using only the actual entry count (e.g. 64 entries = 2 sectors at 4096-byte sectors), but the UEFI spec and Qualcomm's fixgpt both reserve a minimum of 16,384 bytes (4 sectors). This placed the backup entries at the wrong offset, causing a stale partEntriesCrc32 in the backup header.

GPT images only contain the primary table. repairGpt creates the backup via asAlternate(), which now uses Math.max(numPartEntries * partEntrySize, 16384) to respect the UEFI minimum, matching ABL's MIN_PARTITION_ARRAY_SIZE = 0x4000.

Test plan

• repairgpt on LUN 4 (64 entries), verified no CRC mismatch via getactiveslot
• bun test passes
• bun lint passes