Fix Podlang parsing and validation edge cases

[robknight]

This tightens Podlang parsing and validation around several ambiguous or lossy cases.

• Treat reserved words and boolean literals as complete tokens, so identifiers such as record_count, true_flag, and falsehood parse correctly.
• Format REQUEST blocks as whitespace-separated statements so pretty-printed requests can be parsed again.
• Reject duplicate dictionary keys, including duplicates in nested literals, before lowering can discard a value.
• Reject empty REQUEST blocks.
• Reject custom and intro predicate names that collide with native or syntactic-sugar predicates.
• Reject duplicate module aliases before their predicates and records can be mixed in the symbol table.
• Correct container statement argument documentation.

This also removes the unused validation-diagnostics path now that invalid constructs such as empty requests are reported as validation errors, and adds targeted parser, round-trip, validation, and diagnostic coverage.

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