Summary

Fixes #422.

When a method body fails to parse (e.g. an invalid escape '\s' in a string), ANTLR's recovery produced a noisy cluster of diagnostics on top of the original error:

• the lexer reported the bad escape twice — once at the backslash, again at the closing quote when recovery re-entered the string
• control-flow analysis ran on the partial AST and emitted a spurious Expected return statement / Method does not return a value

Both are now suppressed. Example: a class with a single '\s' invalid escape used to report 4 diagnostics; it now reports a single Invalid escape sequence '\s' in string.

Changes

• OuterBlock tracks whether its on-demand block parse produced issues. verify() still runs per-statement verification on the salvaged AST so downstream features (rename, navigation, references) keep working — only verifyControlPath is skipped on parse failure.
• statementsOrErrors() is a new sibling to statements() returning Either[ParseErrors, Seq[Statement]] for callers that want to distinguish a parse failure from a genuinely empty block. statements() retains its existing best-effort Seq behaviour, so existing call-sites are unaffected.
• CollectingErrorListener collapses adjacent Invalid escape sequence diagnostics on the same line — the typical lexer-recovery cascade — while leaving unrelated same-line errors (e.g. two genuine missing ; errors) untouched.

Test plan

• New BlockTest cases cover:
  • the cascade no longer emits Expected return statement / Code path does not return / Method does not return
  • a successful parse still verifies control flow (regression guard)
  • statementsOrErrors returns Left(ParseErrors) on failure, Right(stmts) on success
  • the duplicate Invalid escape sequence diagnostic is collapsed
• Full JVM test suite passes (2398 tests)
• JS cross-build compiles cleanly
• sbt scalafmtCheckAll clean