Lower spread-arm ternaries into per-branch blocks instead of hoisting

changelog.d/ts-ternary-spread-arms.md

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+fix: **Ternaries with spread-literal arms compile from TypeScript cores**: `parsed === null ? q : { ...q, state: "ok", price: parsed }` — and the nested, `!==`, both-arms, argument, object-field, and `x === null ? { ...fallback } : x` spellings — no longer emit Zig that reads the null-narrowing capture before it binds (`use of undeclared identifier`) or evaluates both arms unconditionally; arms that build values statement-by-statement now lower into per-branch blocks feeding a typed temp, so exactly the taken arm runs, and pure-arm ternaries keep their tight `if`/`orelse` expression forms.
+- **Optional switch payloads keep their optional through the capture**: reading a `number | null` payload (directly or via a `const` local) inside its `case` no longer mistypes the value as non-null, which routed `msg.parsed === null ? ... : ...` around the narrowing lowering and emitted `?f64` into `f64` slots.
+- **Early-exit guards narrow like early returns**: `if (r === null) break;` in a parse loop (and the `continue`, labeled, multi-statement, `throw`-exit, and `if (x !== null) { ... } else { break/return }` spellings) now narrows the optional for the rest of the loop body the way tsc's flow analysis does, instead of emitting Zig field access on the still-optional value; `if (msg.kind !== "num") break;` narrows the union payload the same way.
+- **Guard narrowing ends with its block**: a guard's captures no longer leak past the loop body or branch they narrow — reads after the construct see the unnarrowed value again (matching tsc, whose exit path may bypass the guard) instead of referencing an out-of-scope Zig capture.
+- **Early switch-clause breaks stop the build**: an unlabeled `break` that exits a `switch` from inside a clause body now teaches at transpile time — Zig's `break` binds loops, so the old emission jumped past the enclosing loop instead of resuming after the switch.
+- **A redundant kind guard no longer un-optionals a switch payload**: scoped kind-narrowing now restores the still-optional markers alongside the substitutions it snapshots, so `const marker = msg.kind === "got" ? 1 : 2;` inside `case "got":` no longer leaves a `number | null` payload typed non-null for the rest of the clause (which emitted `if (parsed != null) parsed + marker else 0` — invalid Zig operands on the `?f64`).
+- **Inferred locals from narrowed ternaries value non-optional**: `const picked = q === null ? { ...fallback, price: 0 } : q;` (either polarity, no `: Quote` annotation) now types the local by the arm the condition narrows — `Quote`, exactly as tsc infers — instead of the raw optional, which declared a `?Quote` temporary that failed Zig compilation at its first non-optional use (`expected type 'Quote', found '?Quote'`).
+- **A redundant nested `switch` on the same subject hands back the outer capture**: the arm cleanup now repopulates its narrowing maps from the snapshot instead of only deleting the arm's additions, so an inner arm's capture that OVERWROTE the outer arm's entry no longer leaks into the continuation after the inner switch (which emitted the inner capture name after its Zig block had closed — `use of undeclared identifier`).
+- **Else-if chains keep post-if narrowing**: `if (x === null) return -1; else if (flag) { n = 2; } return x.v + n;` — and the else-if-else, chained else-if-else-if, and `!==`-polarity spellings — now narrow `x` after the statement like the plain-else form does; the else-if emission path returned before applying the post-if narrowing, so the fall-through read landed on the still-optional value (`optional type does not support field access`).
+- **Reassigned `let` bindings never fuse into a `const`**: `let p = next(i); if (p === null) continue; p = { ...p, v: 10 };` no longer fuses the declaration and guard into `const p = next(i) orelse continue;` (Zig: `cannot assign to constant`); the binding stays a `var`, the guard keeps its plain null test, and later reads unwrap the live variable — which the assignment path keeps narrowed across provably non-null writes.
+- **A branch that widens a narrowed optional stays widened past the merge**: `if (p === null) return -1; if (flag) { p = null; } if (p === null) return 0;` — the branch-exit restore that keeps narrowing CONTAINED (additions inside a branch die at its exit) no longer also resurrects a narrow the branch killed by assigning null (or a fresh optional-returning call); kills now re-apply after every branch, switch-arm, and kind-guard exit and propagate through nested blocks, so the post-merge re-check tests the live value instead of emitting `p.? == null` (Zig: `comparison of 'f64' with null`). The merge is conservative — a kill on any path that can reach the merge drops the narrow, and the re-check tsc demands anyway always compiles.
+- **Compound-guard branches keep those kills dead too**: `if (r !== null && r > 0) { p = null; }` — where the branch emits under the chain's `.?` substitutions — no longer resurrects p's killed narrow when that substitution scope restores its snapshot (its restore ran after the branch re-applied the kill); the scope now rides the same kill-frame protocol as branch and switch-arm exits, and so does the chain-condition emitter, so every full-map narrowing restore in the emitter re-deletes killed entries on exit.
+- **A kill on an always-exiting branch stays off the surviving flow**: `if (p.v < 0) { p = null; return -1; } return p.v;` inside a null guard — tsc keeps `p` narrowed at the second return because the killing branch left the function, and the emitter now agrees: a branch that always returns (or throws uncaught) drops its kills at the merge instead of deleting the narrow the surviving read depends on (which emitted field access straight onto the `?P`). Kills on paths that resume inside the function — fall-through arms, `break`/`continue` guards in loops, throws caught by an enclosing `try` — still merge outward and drive the post-merge re-check.
+- **A guard in a lifted callback covers the trailing return it precedes**: `xs.map((p) => { if (p === null) throw bad; return p.v; })` lifts the callback as its statement prefix plus the trailing return's expression, and the prefix's narrowing scope closed before that expression emitted — the read landed on the raw `?P` (`optional type does not support field access`); prefix and trailing expression now share one flow scope, and the scope still closes before the callback's siblings in the emitted loop body.