Stream-Aware Vocabulary Demands in Singapore Secondary Tamil Textbooks: A Morphology- and Multiword-Unit-Sensitive Corpus Analysis, with a Textbook-Faithful GenAI Item Benchmark

Vocabulary learning research increasingly calls for curriculum-aligned evidence and methods that reflect how vocabulary is selected, sequenced, and assessed in real educational systems. However, stream-differentiated curricular pathways may entail distinct lexical demands that are rarely quantified for morphologically rich languages such as Tamil, where surface-form variation and digitization artifacts complicate textbook profiling. This study reports a stream-aware corpus analysis of Singapore Secondary 3 Tamil materials (Express, Higher Tamil, Normal (Academic), Normal (Technical)) using a transparent entry-extraction workflow from list-structured PDF resources. We introduce a morphology- and multiword-unit (MWU)–sensitive profiling approach that (i) operationalizes the unit of analysis as list entries (single-word or multiword), (ii) applies conservative Unicode normalization to reduce Tamil PDF segmentation artifacts, and (iii) avoids reliance on opaque lemmatization, treating results as auditable derived indicators rather than definitive lexical-family counts. The corpus comprises 36 pages and yields 1911 extracted entries (1784 unique in the union of streams; 1871 unique when counted within streams), with MWUs comprising 47.7–57.4% of extracted entries and typical MWUs spanning 2–4 words. Between-stream overlap of unique entries is low (pairwise Jaccard 0.010–0.043), and within-stream recycling across pages is limited (0.9–2.1% of within-stream unique entries repeated on ≥2 pages), suggesting substantial stream-specific instructional emphasis and limited built-in recycling in vocabulary-list components. As an innovation component aligned with emerging GenAI practices, we specify and pilot a textbook-faithful item-generation benchmark in which vocabulary practice items are generated under retrieval constraints and evaluated via transparent, classroom-oriented quality screens (faithfulness/grounding, morphological correctness, appropriateness, distractor plausibility, and bilingual gloss quality). The paper concludes with practical outputs for stream-aligned scope-and-sequence planning and a reproducible pipeline that can be extended as additional materials become available.