fix(types,parser): TS conformance Track-1 continuation — symbol member modeling + ParseError cluster

[nickna]

Summary

Continuation of epic #99 / #80 Track-1 (TS conformance). Picks up where PR #1258 left off, per the follow-up comment on #99.

Round 1: symbol member modeling + ParseError cluster

• Parser: extends feat: parse dense .d.ts — es5/es2015.core lib files now parse (#99 Phase A) #168's type-position hardening to value/class position — computed well-known-symbol keys ([Symbol.iterator], [Symbol()], ...) on class fields, interface members, and inline object-type members now allow an omitted type annotation / return type (implicit any), matching identifier-keyed members. Also accepts Symbol (and other contextual keywords) as a namespace name and in typeof-type-position dotted names.
• Checker: object literals and interfaces were collapsing every computed well-known-symbol member into one merged symbol index signature, losing per-symbol precision — this produced spurious errors reading/calling/writing a specific symbol member, and spurious errors assigning an object literal with several distinct symbol members to a matching interface. Each well-known-symbol member is now modeled under its canonical @@name (matching how interfaces/classes already do), with the index get/set paths checking that named member first.
• Three smaller bugs found by the same test cluster: interface string-index-signature compatibility (TS2411) wrongly flagged symbol-keyed members; declare const x: unique symbol; wrongly required a Symbol() initializer; typeof-narrowing didn't recognize numeric/string enum members, and a union typeof-guard with no matching member defined the narrowed binding as a literal null type instead of never (surfaced as a bogus "undefined variable").

Round 2: the remaining ParseError long tail

Seven unrelated, independently-scoped parser gaps found while clearing what was left of the ParseError bucket:

• Object-type/interface members separated only by a newline now use the same ASI fallback as top-level interface members.
• A parenthesized member/index expression is a valid assignment target for =, compound, and logical-assignment operators ((a.b) &&= v) — unwrap parens before dispatch.
• async is a contextual keyword: only commit to parsing an async function/arrow when it's immediately followed by function/</(; otherwise it's a plain identifier (if (async)).
• import.meta used as a statement was routed to the static import-declaration parser, which only excluded dynamic-import's import(, not import..
• export { x as y } (bare specifier list) inside declare global/declare module bodies was unhandled.
• export * as ns from './mod' (ES2020 namespace re-export) wasn't recognized.
• An untagged template with an invalid escape sequence is a real but recoverable syntax error (TS1125) — the parser now substitutes an empty string and records the offending line instead of aborting the whole file's parse; the checker reports TS1125 there. Tagged templates are unaffected.

Result: TS conformance subset baseline 188 → 207 Pass, 24 → 0 ParseError, 0 regressions. The ParseError bucket for this subset is now empty.

Test plan

• dotnet test (SharpTS.Tests) — 15452/15452 passing (both rounds)
• SharpTS.Test262 interpreted + compiled baselines — 0 failures (both rounds)
• SharpTS.TypeScriptConformance baseline — 207 Pass / 71 Fail / 0 ParseError / 17 Skipped / 1 TypeCheckError, 0 regressions vs. committed baseline
• Posted progress comment on TS Conformance Phase 1.5 (deferred): load tsc lib.d.ts files into type checker #99