A fan-in node (>1 input) whose colliding sources hide an unnamed configuration
axis is now illegal at construction, and the definition view renders each fan-in
input as a source-derived recursive-signature identifier instead of the
positional, meaningless #A/#B.
The root defect was sma_cross: two Sma into a Sub with unaliased length params,
rendering sma_cross() -> (cross) with two indistinguishable [SMA] and
[Sub(#A,#B)] — the author meant fast vs slow but the identity hid it. Now it
renders sma_cross(fast:i64, slow:i64) with [SMA(fast)]/[SMA(slow)] and
[Sub(#Sf,#Ss)].
Shape (single source of truth for "signature" shared by engine + CLI):
- aura-engine signature_of(): a node's recursive authoring identity — type
initial + alias initials + each wired input's signature (recursing into
interior leaves, stopping at named roles/composites at their initial).
- aura-cli leaf_label/fan_in_identifiers: shortest sibling-unique prefix of a
source's signature, never below its base (type + alias initials); a role-fed
slot renders its name verbatim (#price); equal-signature interchangeable
inputs keep the positional letter.
- aura-engine IndistinguishableFanIn: a signature collision is a fault only when
a colliding source carries an unaliased param (the unnamed axis); param-less
interchangeable sources (fan_composite's Pass/Pass) stay legal.
Param-aware criterion (refined during design): keeps the blast radius to the
param-bearing alias-less fixtures (sma_cross CLI + engine, the nested fast_slow);
fan_composite and the hand-wired flat fixtures stay green.
Placement decision (deviates from the plan): the constraint runs as a structural
pre-pass (check_fan_in_distinguishability) at the head of compile_with_params,
BEFORE the param-arity gate — not inside inline_composite as the plan drafted.
Reason: the no-param compile() of a param-bearing composite would hit ParamArity
first and preempt the fault; the spec mandates a structural check needing no
param values, so the pre-pass is the spec-aligned home. Alias-validity ordering
(BadInteriorIndex before the fan-in fault) is preserved at the pre-pass head.
C23 untouched: the check is construction-phase; the compilat stays name-free
(compiled_view_golden byte-stable). Ledger C9 carries the refinement.
Known debt (non-gating): the alias-index validity check now exists in both
inline_composite and the pre-pass (the pre-pass reaches every composite first,
making inline_composite's copy effectively dead). Left for a separate tidy — a
5-line, correctness-neutral removal with its own test surface.
closes#44
Brummel
3f4d756ed2