Share the exact-path transcript reader

Delegate ordinary transcript reads to the same exact-path reader used by
stitching. Let that reader own parsing and its file handle, removing the
separate parser layer and duplicated open/close path.

Keep ordinary privacy, filename and group fallback at the existing entry
point. Require explicit read parameters and preserve full/bounded reads,
EOF handling, BOM, newline, multiline and mention behavior. Compared 28
full/bounded read inputs and checked retained message contents.

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