tokenizer: fix bos_token to <|im_start|> (was duplicate-keyed null)

tokenizer: set bos_token=<|im_start|> (matches model config bos_token_id=151644)

tokenizer: set bos_token=<|im_start|> (matches model config bos_token_id=151644)

README: report 50k training steps (matches truncated log + SWA window)

Initial release: SWA-averaged Stage-1 endpoint (steps 46k-50k, 1k stride)

initial commit