spec(0.10.0): §13.1 — a branching_delta row exists only for a template with a branching entry in BOTH documents; a one-sided template has no row, never a zero entropy

[coderoast-dev]

What

SPEC.md §13.1 showed branching_delta in its example and the schema fixed a row's shape, but nothing said which templates get a row. This PR adds a paragraph after the §13.1 example:

branching_delta rows. A producer MUST emit a branching_delta row only for a template_id that has a behavior.branching entry in BOTH compared documents. A template with an entry on one side only has no row: an absent entry is not reported, never zero entropy (§4.2 does not require an entry for every node, and lets a producer cap the block). A consumer MUST NOT read a missing row, or a template absent from one side's branching, as an entropy of zero.

CHANGELOG.md records it under 0.10.0 (still unreleased) › Changed, and the 0.10.0 headline lists it among the breaking items.

Why

• Zero is false in every case an absent entry can mean. §4.2 does not require branching for every node and lets a producer cap it, so an absent entry means "not observed in that window", "no outgoing edge" (an empty distribution, whose entropy is undefined, not 0) or "observed but not emitted / capped out". The reference producer wrote 0.0 for the missing side, so a template appearing with three equally likely successors reported previous_entropy_bits: 0.0, delta_bits: 1.58 — a shift from a value no window observed.
• The consumer half is stated too, because a consumer accepting documents from any producer must not rebuild the same false shift from a template's absence.
• The array's vacuity (§13.2.1) is unchanged: it is emitted only when one of the remaining rows moved, and unmoved rows survive beside the moved ones.

Change class

Breaking: a producer emitting one-sided rows becomes non-conformant. Under GOVERNANCE.md §2's 0.x rule (PR #15), the editor merges it with no rfc: issue. No schema change: the rule involves both input documents, which no keyword can see. conformance/metalog_validate.py --selftest: 31/31 fixtures pass.

The reference implementation (insight-metalog) adopts the rule in the same release. On its determinism digest the rule removes 45 of 48 branching_delta rows, every one comparing a template the other window never held in branching; the 3 two-sided rows stay.

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