All occurrences of Closure(<sig>) -> (T, !) with a structurally identical
value-signature now share one inferred error-set node; every bare-!
closure literal of that shape unions its escape tags in, and a
`try slot(x)` against any matching-shape slot widens the caller's named
set against that union. This closes the gap where a slot call (no static
function name) skipped the widening check entirely.
- shape_inferred_sets keyed by closureShapeKey (params + value-return via
mangleTypeName, error slot excluded) so bare-!, non-failable, .function
and .closure of one value-sig collapse to a single key.
- convergeClosureShapeSets pre-pass (lowerRoot Pass 1d', after the
name-keyed convergeInferredErrorSets): collectClosureShapes walks fn
bodies through lambda boundaries; recordClosureShape resolves each
concrete bare-! literal's shape and unions its raises (+ try named_fn()
edges via calleeEscapeTags) into the shape node.
- checkEscapeWidening falls back to shapeKeyOfCallee for bare-! slot calls
(computed from the callee expr's .function/.closure type). Empty union
is silently allowed (sub-feature 6).
Scope: concrete shapes only (generic lambdas skipped); closure-to-closure
try edges are not fix-pointed (under-approximation = a missed diagnostic,
never a miscompile).
Tests: 1041 (positive — union composes, runs), 1042 (reject — two
widening diagnostics, exit 1).
