Gate review: the fix left three adapter-private copies of `_coerce_float_extra` and the
0.3/2.0/1.0/4.0 cadence literals in three files. The parser and the cadence constants now
live on BasePlatformAdapter beside the delay attrs they configure; WhatsApp and Weixin call
`_configure_text_batch_delays()`, Telegram reads the same constants through its env helper.
The clamp test is parametrized over both adapters and the Weixin docs name the ceilings.
530fdb5867