Fix #784, #781, #780, #779: destructuring default/rest/cover-grammar gaps

[nickna]

Completes a cluster of array/object destructuring conformance gaps. Both interpreter and compiled modes; a representative compiled program passes --verify. Rebased onto current main (the build break that #769×#786 introduced was already repaired on main by 3564041, so no build-fix is included here).

Fixes

#784 — defaults apply on null instead of undefined-only

All six destructuring desugaring sites used access ?? default (Expr.NullishCoalescing), which also triggers on null. Replaced with a shared DefaultIfUndefined helper — v = access; v === undefined ? default : v — that spills the access so it is read exactly once (getter-safe) while keeping the default lazy. Reuses existing nodes, so no new emitter/visitor surface.

Also fixes a latent #754 compiled bug this exposed: an assignment-destructuring value-type default over an absent element ([c = 5] = arr, ({z: d = 5} = {})) produced NaN. The lowered spill temps live inside an Expr.DestructuringAssign expression, so they escaped the whole-body numeric-slot taint pass and got an unboxed double slot that coerced the runtime undefined sentinel. Flagged in VisitDestructuringAssign.

#781 — typed-array rest binds a typed array (interpreter)

const [a, ...rest] = new Uint8Array([1,2,3]) bound a Uint8Array slice (Array.isArray(rest) === false). NormalizeArrayDestructureSource now materializes typed arrays/buffers into a fresh SharpTSArray (they expose index access but no [Symbol.iterator] in this runtime, so they cannot route through GetIterableElements). Compiled mode is blocked by #782 (typed-array construction).

#780 — object shorthand-with-default {a = 5} unparsed

The object-literal parser rejected the cover grammar. Now accepted (stored as { a: (a = 5) }) so it round-trips to the #754 assignment-destructuring lowering. An IsShorthandDefault flag on Expr.Property keeps a pure-expression {a = 5} a type error (TS1312), matching tsc, while {a: x = 5} stays valid.

#779 — nested pattern with a default in assignment destructuring

[[a] = []] = [[1]] / ({p: {x} = {}} = ...) were a parse error. Expr.DestructuringAssign now retains the un-lowered (RawTarget, RawDefault); the outer pattern walk re-lowers the inner pattern against the defaulted access.

#753 — string rest → array

Already fixed in main by #785; verified in both modes and covered by tests.

Tests

14 new destructuring cases (both modes where applicable): undefined-only defaults (declaration + assignment), value-type default over absent (the compiled NaN guard), laziness + single-evaluation, typed-array rest, object shorthand-default, the pure-expression cover-grammar error, and nested array/object patterns with defaults.

Follow-ups filed

• Object destructuring with a default over a source lacking the property reports spurious 'Property does not exist' (declaration + assignment) #796 — object destructuring with a default over a source lacking the property reports a spurious "Property does not exist" (pre-existing; blocks the empty-source examples of Object shorthand-with-default {a = 5} is rejected by the object-literal parser (blocks that form of assignment destructuring) #780/Assignment destructuring: nested pattern WITH a default ([[a] = []]) is unsupported #779). Same "binding-pattern contextual typing" root cause as Nested destructuring over a mixed (non-tuple) array literal fails type-checking (const [[m], n] = [[7], 8]) #783.
• Nested destructuring over a mixed (non-tuple) array literal fails type-checking (const [[m], n] = [[7], 8]) #783 — left open with findings: a tuple-inference attempt fixed the leaf case but not the headline nested case (the inferred tuple is lost on the synthetic temp) and changed over-destructure behavior, so it was reverted.