add super.Arena to batch value copies in sbuf.Array

[nishantmehta]

Closes #7064.

Problem

sbuf.Array.Write copies every value with Value.Copy → bytes.Clone, so it
allocates once per non-native value. Reading a large input into an Array
allocates proportionally to the value count — BenchmarkReadBSUP (10M values)
does ~10M allocations.

Change

Add a chunked super.Arena and have sbuf.Array copy through it:

• value payloads are appended into 64 KiB chunks;
• a chunk is never reallocated once it holds a value, so values copied
  earlier remain valid as later ones are appended (when a chunk fills, a fresh
  one is allocated and the old one is kept alive by the values referencing it);
• native values carry their payload inline and are returned unchanged (no
  allocation), matching Value.Copy;
• a value with no bytes (including a null) is returned with its bytes
  unchanged, preserving the nil-vs-empty distinction bytes.Clone would;
• the returned bytes use a 3-index slice so appends can't scribble into the
  arena's spare capacity.

Result

BenchmarkReadBSUP:

	before	after	delta
allocs/op	10,000,416	904	−99.99%
B/op	1.63 Gi	1.59 Gi	−2.6%

ns/op is unchanged (the benchmark is GC/bandwidth-bound); the win is the
collapse in allocation count and the resulting GC pressure on large reads.

Testing

• TestArenaCopy covers native values, source-buffer independence, null
  (nil-bytes) preservation, values spanning many chunks, and a value larger
  than one chunk.
• go test -short ./... passes across the repo (round-trips unchanged).