enhance(config, typewriter)!: Switch DelayDuration to humantime format

[JeanMertz]

The previous custom duration format ("3" for 3 ms, "5m" for 5 ms,
"1u" for 1 µs) was non-standard and undiscoverable. This replaces it
with [humantime]-compatible strings ("3ms", "500us", "1s",
"0s") across the entire TypewriterConfig.

Default values in #[setting(...)] annotations are updated accordingly:
text_delay from "3" → "3ms", code_delay from "500u" →
"500us", and max_latency from "0" → "0s".

Custom Serialize/Deserialize implementations replace the derived
ones so that values round-trip as humantime strings. A backwards-compat
visit_map path accepts the old {secs, nanos} shape that stored
conversations may still carry, so existing data keeps loading without a
migration.

The Schematic impl is now handwritten to expose a descriptive string
type in the generated schema. Display and FromStr are also
implemented, both delegating to humantime.

The default persona config is updated to set typewriter.max_latency = "200ms" using the new format.

BREAKING CHANGE: Typewriter delay format changed

Any jp config file that sets text_delay, code_delay, or
max_latency using the old shorthand ("3", "5m", "1u") will fail
to parse. Update those values to humantime strings, e.g.:

text_delay  = "3ms"
code_delay  = "500us"
max_latency = "200ms"