Errors: render switch, throw, try, and do-while SyntaxErrors

Parent: #899

Problem

Five statement-parser exits discard grammar context in parseDoWhile, parseSwitch, parseThrow, and parseTry: four return a bare UnexpectedToken and one returns a bare ExpectedToken.

Executed on current main (40e64ef), nine representative eval probes expose internal tags. Raw JavaScriptCore 625.1.22 reports:

• duplicate default and an invalid switch-body item as an unexpected exact token followed by Expected '}' to end a body of a 'switch'.
• a line terminator after throw as Cannot have a newline after 'throw'.
• try {} at EOF as Unexpected end of script, and a non-EOF follower as an unexpected exact token followed by Try statements must have at least a catch or finally block.
• a missing while after do as Unexpected end of script at EOF, or an unexpected exact token followed by Expected 'while' to end a do-while loop.

Implementation

• Retain the current switch-body token, try follower, and do-while follower.
• Attach the restricted-production failure to its consumed throw keyword.
• Let the existing EOF diagnostic mapping produce Unexpected end of script.
• Match raw JavaScriptCore prose and preserve compact ParseError categories.
• Add no allocation or source scan to successful statement parsing.

Verification

• Pin reason, message, and byte offset for all nine probes.
• Add a focused production eval case for each message/token family.
• Compare exact-parent switch, throw, try, and do-while test262 subtrees with nonzero denominators and report flips.

Implemented and pushed to main in 029a701.

• Duplicate/invalid switch clauses and non-EOF try/do-while followers now retain their exact tokens.
• A newline after throw is attached to the consumed keyword with JavaScriptCore prose.
• EOF after try {} or do {} reuses the existing end-of-script classifier.
• Successful statement parsing adds no allocation or source scan.

Validation:

• parser module: 234/234
• focused frontend boundary: 1/1
• raw JavaScriptCore differential: 9/9 byte-identical messages
• exact-parent test262 comparison: switch 111/111, throw 14/14, try 201/201, do-while 36/36 before and after (0 flips)

This reduces direct return ParseError.UnexpectedToken; sites from 68 to 64 and removes the adjacent bare ExpectedToken exit.