test(layer): cover load's failure modes and same-date index ordering

load previously had zero coverage despite being the module's only fallible-
I/O function and the one place the brief's literal code would have let an
exception escape on this sexplib version (Failure instead of Parse_error).
Add cases for a missing file, a malformed/unterminated sexp, an invalid slug
embedded in an otherwise well-formed layer, and a success round-trip.

Also add a same-date index case with three entries inserted out of slug
order, pinning that the by-date index accumulates rather than overwrites and
returns buckets in canonical order regardless of insertion order -- the
previous two-entry fixture only ever touched bucket sizes 0 and 1.

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