fix(semantic): typed-return concise arrow opens a scope and declares params (#60)

[ericsssan]

Fixes #60.

Problem

The parseParenthesized typed-return arrow branch ((params): T => body) built the arrow_fn node without calling emitScopeOpen/emitParamDeclaresFromRange. So for untyped params with an explicit return type, the arrow opened no scope and declared no parameters — they were absent from the scope tree entirely, breaking every scope-aware analysis (no-unused-vars, no-shadow, no-undef, redeclaration, …).

const f = (a, b): number => a + b;   // a, b had NO symbols, no arrow scope
const g = (a, b = a): string => b;   // same

The typed-param path ((a: T): U =>, via looksLikeTsArrowParams) and the empty-param path already worked.

Fix

Open the arrow scope and declare the params before parsing the body, mirroring the other arrow paths, and reuse the function-level param_ev_mark to re-home parameter-default references into the arrow scope (#56 parity). The body then parses inside the scope so its references resolve to the params.

This branch has a speculative backtrack for the cond ? (a): T => body : alt conditional-consequent ambiguity. Scope events are emitted only when no backtrack is possible (!saved_cc) — otherwise they would orphan on the backtrack — so that rare ambiguous case keeps its prior (no-scope) behavior, and there are no events to undo on the backtrack path.

Validation

• Full suite green. New tests: typed-return arrow declares its params in an arrow_function scope and the body resolves to them; default re-homes to the param (semantic: arrow-function default value does not resolve sibling/self parameters (deferred param-declare ordering) #56 parity); js_ts-mode parity.
• TypeScript conformance identical to baseline: must-parse 17910/17913, must-reject 1210/1223.
• Semantic sweep over 19,233 files: 0 crashes, with the expected deltas vs main — +45 arrow scopes, +47 parameter symbols (params now declared), and −47 references (the spurious param-name reads from the cover parse are now correctly cancelled as declarations rather than counted as value reads).