sysview: per-family RTT buffer defaults, 65536 on RAM-rich parts

The dual-role dogfood measured the 4096 fallback losing 96.7% of ISR exit
contexts on mimxrt1064_evk and 99.1% on metro_m4_express; a local-board run
measured stm32u575nucleo losing ~19% of the whole stream under a 12 s
cdc_msc workload. The default was too small for any real capture on fast
parts and every user had to rediscover that per board.

Families now declare SYSVIEW_BUFFER_SIZE_DEFAULT in family.cmake (the
SYSVIEW_RAM_BASE_DEFAULT pattern): imxrt, nrf, rp2040, samd5x_e5x, stm32f4,
stm32f7, stm32h7 and stm32u5 set 65536, the measured-safe value.
-DSYSVIEW_BUFFER_SIZE still beats everything; small-RAM parts keep the 4096
fallback and their per-board overrides (stm32f072disco's 2048 verified
still honoured). lpc55 measured 34 lost events on the same route, an order
of magnitude lighter, so it keeps the fallback rather than inheriting a
default on thin evidence.

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