TSemanticAnalyser walks the AST, resolves all identifiers via the symbol
table, annotates every expression node with ResolvedType, and type-checks
assignments and procedure calls. Raises ESemanticError with source position
on undeclared identifiers, type mismatches, duplicate declarations, and
unknown types. TProgram now owns the TSymbolTable after analysis so
ResolvedType pointers remain valid for the lifetime of the AST.
Also adds the 'div' keyword for integer division throughout the pipeline
(lexer, parser, semantic). 26 new FPCUnit tests, 126 total, all passing.
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