Savestate: compact BlockAllocator serialization (section v2)

[Arkadyzja]

What

Bumps the BlockAllocator savestate section to v2 with a compact per-block serialization form, and zero-pads block tags at write time instead of on every save.

Why

Serializing the kernel memory block lists (userMemory, kernelMemory, volatileMemory) currently pays two per-block costs on every save:

1. A full Section("Block", 1) header per block — games keep on the order of a thousand blocks alive, so this is a thousand section headers of pure overhead in time and payload bytes.
2. A strlen + memset per block to zero the tag padding before writing it (needed because truncate_cpy leaves the padding uninitialized).

How

• Block's constructor and SetAllocated() now memset the whole tag before writing it, so tag padding is always zero by construction and can be stored raw. Every path that writes a tag clears it first, so uninitialized bytes still never reach the stream.
• The v2 form stores each block as raw start, size, taken, tag — no per-block section machinery, no per-save tag scrubbing.

Compatibility

• v1 states load exactly as before — the old per-block-Section read path is kept unchanged, selected by the section version.
• States written by this build won't load on older builds — standard section-bump consequence.