The residency probe was an inherent method on the concrete M1FieldSource, so a Box<dyn Source> consumer could not read it without downcasting (the adjacent friction noted in #95). Hoist it onto the Source trait as resident_records(&self) -> Option<usize> with a default of None, mirroring bounds()'s Option-for-unknown idiom: None = "does not report" (the eager VecSource, which has no streaming ring), distinct from Some(0) = "reports, holds none" (e.g. exhausted).
M1FieldSource overrides it to return Some(chunk len). Callers updated (streaming_seam, the two deep-dive fieldtest bins). Tests: a hermetic VecSource default-is-None check, and a &dyn Source probe in the gated residency test proving M1FieldSource residency is readable without a downcast.
refs #95
Brummel
f66036bd61