fix(hy-check): reject untyped nil local declarations (var x = nil / x := nil / let x = nil) with HYB-E4003, the N1 untyped leg (fixes #672)

[milkingndb-oss]

Fixes #672

The bug

The N1 nil check at both local-declaration check sites was ascription-gated (if let Some(t) = &l.ty). With no ascription - var x = nil / x := nil / let x = nil - there is no branch to take, shape_of_expr(nil) is TypeShape::Unknown, and the checker accepted the declaration with 0 diagnostics. The emitter then emitted var x = nil / x := nil verbatim, which Go rejects outright: use of untyped nil in variable declaration. Accepted-but-unbuildable - GUARANTEES row 13 (BUILDABLE). Full repro table + controls in #672.

The fix

• New registry code HYB-E4003 untyped-nil-declaration (rule N, tier0 N1, category nil-safety): nil has exactly two homes (N1) - error and the foreign-Go perimeter (N2) - and both must be visible in a DECLARED type; an untyped nil can never name either. Three fix routes taught: ascribe the nil home (var e error = nil); declare without initializer for the zero value (N3: var ch chan int); option[T] for absence. A new code rather than reusing E4001 because E4001's message and fix-its are built around a named declared type (nil is not valid for {ty}) - here the defect is precisely that no type exists.
• loans.rs process_local: else-branch on the E4001 ascription gate emits E4003 for bare-nil initializers of var/let/:= kinds (the flat walk covers loop/match-arm/if-block bodies).
• closures.rs check_stmt_boundaries: the same else-branch for closure bodies (the flat walk skips FnLit bodies; this walk owns them).
• Local const c = nil is left alone: Go's error there is nil is not constant - the E6003 constant-expression guard's locus (Soundness & buildability gaps in make/new shape resolution, closure builtin validation, bare chan T expressions, and file-level const forward references #640, PR fix(hy-check): reject nil consts and support f32/f64/char and string aliases in const_init.rs #671). The two loci do not double-fire (l.kind != LocalKind::Const in both branches).

Verification

gate	result
cargo test --workspace	1150 passed, 0 failed (the one spec_t3_strings failure is a pre-existing CRLF-in-checkout issue - reproduces on clean 0ca08c2, unrelated)
new unit pins hy-check/tests/untyped_nil_decl.rs	13/13: all four spellings (var/:=/let/parallel a, b := nil, 1), closure body, loop body, match arm, blank _; controls stay accepted (ascribed var e error = nil, bare nil as call argument, ch == nil compare, zero-value declarations); local const c = nil does not fire E4003
corpus harness	80/80 verdict agreement, tripwire not fired (new fixture untyped-nil-decl-reject.hy pins REJECT N1 + exact diagnostic HYB-E4003, registered additively in VERDICTS.md)
differential reject-side	57/57
hyb explain HYB-E4003	renders from the registry with the three fix routes

Corpus/examples impact: zero - no existing corpus/example file contains an untyped-nil declaration (grepped); the fix is regression-safe by construction.

Claim check

Searched open issues + PRs for untyped nil, var x = nil, use of untyped nil, E4003: no claim on this locus. #640/#670/#671 are the const-nil family (E6003, expr_is_constant_inner); corpus 28 is composite-literal nested nils; the ascribed E4001 sites are unchanged.