docs(asapv1): DDSketch has one encoding, carrying the negative store and zero count

[GordonYuanyc]

Closes #186. Part of #179.

What the spec says

• DDSketch (0x05 0x00) has one encoding: metadata_version is 1 and the payload always has eight fields, [counts, offset, sum, min, max, negative_counts, negative_offset, zero_count].
• negative_counts / negative_offset follow the positive store's rules; an empty negative store sits at negative_offset == 0.
• Decode rules 4–7 cover both stores: empty form, i32 index span, and the checked total sum(counts) + sum(negative_counts) + zero_count.
• Rule 7: every non-empty state has finite scalars with min <= max; when sum(negative_counts) == 0 and zero_count == 0, also 0 < min and sum >= min.
• Section 4 and Q-DERIVED name the new fields.

Choices beyond the issue text

• metadata_version is 1, as for every other kind.
• The three fields follow max, the same order as today's signed payload.
• The empty negative store is pinned at offset 0, as the positive store is.

Not in this PR

Rust and Go encoders and golden files change in #195. No DDSketch golden exists in sketchlib-golden-bytes yet.

Only docs/asapv1_wire_format.md changes.

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