fix(chart): render GitHub App ids as integers

[BryanFRD]

Closes #560

• Chart: a numeric appId goes through an lfsx.appId helper that turns Helm's float64 back into an integer before quoting, for the main forge and each named forge. A string id is left as is. On main, appId: 1234567 renders "1.234567e+06"; with this, "1234567" whether given as a number or a string. Linted and rendered with helm 3.16.
• Review nit from refactor(storage): reuse Survey in the indexed sweep and split the staging walk #550: sweep_indexed binds own_prefix(ns) once, so the listing and the survey visibly share it.
• Review nit from fix(cli): report no saving instead of overflowing when compression grows the objects #552: summary::verify writes with writeln! instead of push_str(&format!(..)).

CLI and collection tests and clippy pass.

[ferrfleet]

Nothing blocking.

• Chart: Helm parses values.yaml integers as float64, so quote turned large app ids into 1.234567e+06. The lfsx.appId helper only casts float64 and lets --set int64 values and strings through unchanged, so it's safer than a blanket | int64, which would quietly turn a bad string into 0.
• summary.rs: writeln! into the String is the idiomatic replacement for push_str(&format!(..)) (clippy format_push_string).
• collect.rs: computes the prefix once. No change in behaviour.