refactor: one word per concept in the configuration chain

[thodson-usgs]

Why

Follow-up to #396, which was documentation-only: this is the code half, plus a
scan of every CONTEXT.md term against the package.

_resolve returned (raw, source, tier). The glossary's source — the
ordered category a value came from — was the third element; the second was
the origin label, the exact origin such as $API_USGS_RETRIES or a path
to the config file. One word, two grains, in one signature. And tier was our
own vocabulary misapplied: ADR 0010 uses tier for the scope axis, and ADR
0011's rungs are ladder positions.

#405 (merged) supplies the rule that licenses touching identifiers: terms we
invented take one spelling everywhere, identifiers included. This PR is that
rule applied to the configuration chain.

What changed

_resolve now returns (raw, label, source):

concept	was	is
source — the ordered category (block / environment / file / built-in)	tier	source
origin label — the exact origin, printed by show_configuration()	source	label
the helpers that build origin labels	_source_label, _env_source_label, BaseConfiguration._source	_origin_label, _env_label, ._label

label is not a coinage: _show_adapter_overrides already used it, and
_parse_int documented its parameter as "Human-readable origin". Prose
follows the code: the show_configuration docstring and the user guide said
"source" where they describe an origin label.

Behaviour unchanged, mechanically

Applying the rename to main's AST — docstrings stripped — reproduces this
branch's AST exactly for both renamed modules, and the other touched files are
identical in code terms. No string constant changed except two docstrings;
show_configuration()'s output is byte-identical. mypy --strict is clean,
and all 1139 offline tests pass.

Stripping docstrings is also that check's blind spot, and it bit: the rename
left "a label label" in _resolve's docstring, wrote "source" in three places
where the glossary means scope, and missed tier in
waterdata/configuration.py and four test docstrings. The follow-up commits
fix the wording; each message says what it fixed.

Commits

1. 17f9cdc — the rename.
2. 934d4ed — the wording the rename got wrong.
3. b242704 — ..._the_tiers_below → ..._the_rungs_below, with ADR 0011's
   compliance citation annotated the way ADR 0008 annotated the wateruse
   rename.
4. f5f29ee — two pre-existing docstrings that called a rung a scope.
5. b10fb35 — merge of main after docs: separate the terms we own from the terms the services own #405 landed. One conflict, in CONTEXT.md's
   Origin label entry: kept docs: separate the terms we own from the terms the services own #405's Core terms. marker and first
   sentence, then this branch's statement that the code now matches.
6. 590a683 — one docstring that said "sources below" for what are rungs 4
   and 5.

Raised, not fixed — needs your call

The scan cross-checked all 40 CONTEXT.md terms; everything audits clean (no
tier or rung survives in the package) except one thing: the package says
service in places where the glossary's term is adapter — eleven sites
in the two configuration modules, including "<service default>", which
show_configuration() prints, and base_url's docstring ("the service
passes its own") in a function whose parameter is adapter.

Left alone deliberately: some may be legitimate — a base URL arguably is the
service's, even though the setting is adapter-scoped — so each site needs its
own judgement, and the printed one changes user-visible output, which has no
place in a PR whose whole claim is that behaviour is unchanged.