Add a leaf-name param accessor (getparam) to remove the dotted-key footgun #12

Problem

DataKey.params is keyed by the dotted group.leaf path, so a work_fn must
write key.params["system.N"]. The plain leaf key.params["N"] silently raises
KeyError. This is the single most common point of confusion when writing a
work_fn (and a frequent mistake for code-generating assistants): the dotted
form is implicit — it comes from the [paramsets.system] table nesting, not
from anything the user typed literally — so the natural guess params["N"] is
wrong with no hint as to why.

Proposed enhancement

Add a core accessor that resolves an unambiguous leaf name and reuses the
existing AmbiguousPathKeyError machinery, so the dotted form keeps working
unchanged but the leaf form is also supported and fails loudly:

"""
    getparam(key::DataKey, pk) -> value

Look up `pk` in `key.params`, accepting either an exact key ("system.N" or a
top-level "N") or an unambiguous leaf name ("N" matching "system.N").
Throws AmbiguousPathKeyError if the leaf appears in multiple groups.
"""
function getparam(key::DataKey, pk::AbstractString)
    haskey(key.params, pk) && return key.params[pk]
    matches = [(k, v) for (k, v) in key.params if _split_dotted(k)[2] == pk]
    isempty(matches) && throw(KeyError(pk))
    length(matches) > 1 &&
        throw(AmbiguousPathKeyError(String(pk),
                                    sort([_split_dotted(m[1])[1] for m in matches])))
    return matches[1][2]
end

getparam(key, "N") then works whether the config wrote system.N or a
top-level N, and on a genuine collision (system.N and model.N) it raises
the same actionable error format_path/resolve_path_keys already use, instead
of a silent KeyError.

Use case

Every work_fn in apps/ReducedEnvExperiments.jl indexes params["system.N"],
params["system.chi"], params["strategy.name"], … A leaf accessor removes the
need to memorize the group prefix and converts the most common silent failure
into either a correct lookup or an explicit, fixable error. (Mirrors the
leaf-resolution format_path already does internally.)