BFM-decomp/cookbook/C0218.md

§196 — PUT ON THE CARD WHAT THE TREE ALREADY KNOWS: the fleet's declaration consensus (P31 S54)

The measurement that chose this lever. Wave V's output tokens, attributed by phase:

phase	agents	output tokens	share
draft	70	2,748,778	88.3%
repair	6	262,938	8.4%
reconcile	9	99,986	3.2%

So the plumbing is already cheap — eliminating declaration conflicts outright would recover ~3%. The leverage is in what a drafter must GUESS before its first compile, and the wave averaged 9.3 match_one compiles per agent (37 of 85 agents needed 10 or more). Every guess that can be answered from the tree instead of from a compile is a direct saving.

Two guesses are answered somewhere in the tree and were on nobody's card:

• CALLEE ARITY AND RETURN TYPE. §195-A proved there is no positive tell in the asm — an argument that dies at the call is allocated straight into $aN, so its only def is a plain load and every intervening use reads $aN; the def/use walk cannot decide, in either direction. The prescribed procedure is a two-arity A/B, i.e. an extra compile per ambiguous callee. But some other TU has usually banked a caller already: func_8012BEE8 is declared ('s32', ('s32',)) in 4,674 places fleet-wide.
• GLOBAL TYPE. Every CONFLICTING-EXTERN drop and the whole Reconcile phase exist because N drafters independently invent a spelling for one D_ symbol. The fleet has usually settled it.

tools/decl_prior.py builds the index deterministically (4,162 files → 67,094 symbols, 9,739 with a banked DEFINITION) and build_wave_atlas puts the rows for each target's own symbols on its card, strongest evidence first:

1. DEF — a banked definition's own signature. The function exists; this is its real shape.
2. TU — the destination TU's own declaration. Authoritative for this draft by wave law 2 whatever the fleet says, because that is the file it has to compile in.
3. FLEET — the modal extern spelling with its count, plus its rivals and their counts.

Measured on a control draw: 63 rows over 10 cards, 83% already settled by the destination TU — which is not wasted, because the agent was grepping the TU for each of them by hand. The other 17% is the part with no local answer at all, and that is precisely the §195-A arity guess.

THE GENERAL LAW, third instance in one session. §193-A (seed_ref, the atlas's banked twin), §194-E (tu_ref, the banked neighbour in the destination TU) and now §196 are the same finding: the answer was already computed and the card did not carry it. Before adding an agent, an attempt, or a prompt paragraph, ask what the tree already knows — the card is the cheapest place in the whole pipeline to put a fact, and every field costs zero tokens per wave forever.

What this is NOT. It is not a struct model, and it does not attempt one. docs/actor-struct.md already measured that: feeding the recovered ~154-field actor struct to m2c as --context scored 0 better / 10 same / 2 worse on a 12-function sample — type recovery is comprehension, not a match-rate lever. And p->field is not always byte-equal to *(T *)(p + K) (§183.3: a spelling that forces &D_x materialises one shared base register and perturbs the whole body). Declarations are the part that is both cheap to derive and load-bearing for the GATE; the struct model is a readability project for later, introduced per-adoption behind the byte gate.

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