test(memstore): add tests for Store operations and Entry conversions

[Qalipso]

core/memstore is at 11.0% statement coverage. It is a self-contained, dependency-free package, so it is straightforward to cover.

This adds tests for:

• the Store surface: Set, Get, GetDefault, GetEntry, GetEntryAt, Exists, Remove, Reset, Len, Visit, GetOrSet
• insert vs update semantics of Save (the returned boolean)
• immutability: SetImmutable entries reject a plain Set but accept another SetImmutable
• Entry conversions and their error paths: StringDefault, StringTrim, IntDefault, BoolDefault, TimeDefault, WeekdayDefault, Float64Default
• ErrEntryNotFound formatting and As matching, including via errors.As
• the sized integer getters, including the out-of-range rejection in the unsigned variants

Coverage goes from 11.0% to 59.2%. No non-test code is changed; go vet and go test -race pass.

---

While writing these I ran into what looks like a bug in UintDefault, so I left it uncovered rather than encode the current behaviour in a test:

var maxValue uint64 = math.MaxUint32
if x64 {
    maxValue = math.MaxUint64
}
...
case int:
    if vv < 0 || vv > int(maxValue) {
        return def, e.notFound(uintType)
    }

On a 64-bit platform maxValue is math.MaxUint64, so the conversion int(maxValue) wraps to -1. The comparison becomes vv > -1, which is true for every positive int, so UintDefault returns the default and a not-found error for any plain int value:

var p memstore.Store
p.Set("n", 8)
p.GetUintDefault("n", 0) // returns 0, not 8

The sized variants (Uint8Default and friends) use literal bounds and are not affected. Happy to send a separate PR for it if you'd like — I did not want to mix a behaviour change into a test-only PR.

[CLAassistant]

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