fix(cfg): catch and post-labeled-break segments wrongly unreachable (#57)

[ericsssan]

Fixes #57.

Two segment-reachability bugs in code_path.zig caused no-unreachable false positives. The JS runner reconstructs reachability from the serialized seg_reachable table, so node_reachable (a separate loop-only pass) masked these — they're only visible in the segment graph.

Bug 2 — catch unreachable when the try body ends with return

try { bar(); return; } catch (err) { return err; }   // catch wrongly marked dead

first_throwable_called was set only by explicit throw and return <expr>. A bare throwable call (bar()) + an argless return set it for neither, so the catch was marked dead. Fix: a new throwable event — the parser emits it for Call/Member/New/Yield expressions evaluated in a try body (gated on a new in_try_body flag; a single predictable bool check in addNode), and the CFG's makeFirstThrowablePathInTryBlock makes the catch reachable from the try entry. try_context is now saved/cleared across enterCodePath/exitCodePath, so a throwable in a nested function does NOT make the outer catch reachable.

Bug 1 — break <label> to a labeled block

A: { break A; } foo();   // foo() wrongly marked unreachable

makeBreakLabeled only walked loop_context, so a break to a labeled block found no target. Fix: a LabelBreakContext for non-loop labels — label_open (non-loop) pushes it, label_close pops and merges the break targets with the body's normal exit into the post-label segment. Labeled loops are unchanged.

Validation (against the serialized seg_reachable)

• Catch reachable for call / yield / member-write throwables; still dead for an empty try (try { return; }) and for a throwable in a nested function — matching ESLint.
• foo() reachable after A: { break A; }; still dead after a block that always returns; labeled-loop break unchanged.
• Full suite green; TS conformance 17910/17913 · 1210/1223; babel 1928/1928 · 1548/1548; test262 3966/3966 · 1389/1389; semantic sweep 0 crashes, scope/symbol/ref/diagnostic totals byte-identical to main (the change alters only segment reachability, which the sweep doesn't tally).