Fix silent partial parses and quoted Bash keywords - #8

Problem

unbash could return a clean AST even when it stopped before the end of the source. Given:

safe
fi; recovered

it returned only safe, omitted recovered, and reported no error. Consumers therefore could not tell that the AST did not represent the complete input.

Bash recognizes reserved words only when unquoted. unbash instead checked the processed word value in some paths, so "time" echo became timing syntax rather than a command named time; ANSI-C spellings such as $'if' were similarly classified as keywords.

Changes

Unconsumed top-level tokens now produce source-positioned errors. When a statement separator provides a safe restart point, parsing continues so later statements remain available in the best-effort AST.

Reserved-word recognition now records whether a word is lexically eligible. Quoted, escaped, or expanded spellings remain commands or operands, while ordinary and backslash-newline-spliced keywords retain Bash semantics. Deep recovery follows the same rule.