memory: bound segment growth to prevent makeslice panic on malformed program

[ipezygj]

Problem

Segment.Read/Write grow a segment to fit an offset via IncreaseSegmentSize(offset + 1), which does make([]MemoryValue, newSize) with an unbounded, program-controlled size. A program that reads or writes a huge memory offset makes the Go runtime panic with makeslice: len out of range, which crashes the process — there is no recover() in the VM/runner.

Repro (a program whose bytecode reads offset 0x80000000000001):

panic: runtime error: makeslice: len out of range
  ...IncreaseSegmentSize (pkg/vm/memory/memory.go:160)
  ...Segment.Read (pkg/vm/memory/memory.go:122)
  ...RunStep -> RunUntilPc -> Run

Fix

Reject an offset beyond a generous maxSegmentSize with a clean error in Read/Write, so a malformed program returns an error instead of crashing the host. The limit is intentionally generous (no legitimate execution approaches it) and commented as tunable to the intended memory model.

• make([]MemoryValue, huge) panic → graceful error.
• Adds TestSegmentReadWriteHugeOffsetReturnsError.
• No regression: go test ./pkg/vm/memory/ passes; the repro program now returns memory offset ... exceeds max segment size instead of panicking.