Says a chart is keyed by its content hash alone

[johnnyt]

What

StatifierPersistence.Storage.save_chart/3 keys a chart on
Machine.identity/1's content_hash and nothing else. No tenant,
namespace, or host scope takes part in the key, so two tenants that store
byte-identical charts share one chart row. The moduledoc said where the
hash comes from but never what follows from it, and the question a
multi-tenant host asks first was answered nowhere in the package.

Three paragraphs on Storage's moduledoc now say it:

1. The keying. The hash is a content address; saving the same one twice
   is :ok and does not duplicate the row (the Adapter.save_chart/2
   contract). The hash answers which chart these bytes are, never who
   stored them.
2. The tenant consequence. A multi-tenant host tenant-qualifies its own
   per-chart rows, in its own tables. This package stores nothing per
   tenant; a run's opaque metadata map (ADR-0006) is where a host tags a
   run with the scope it already keys its own tables by.
3. The namespace question is upstream's. Folding a namespace into the
   hash would change what a chart's identity is, which is
   Statifier.Machine.identity/1's question - statifier-ex's contract, not
   this package's. Nothing here proposes it and no option to do it is
   offered.

InMemory's moduledoc mirrors the first two on its chart map, and the
README says it once at the end of "Listing runs by host scope", which is
where a multi-tenant host reads about scope.

Zero code change

Machine-checked, not asserted: stripping every @moduledoc/@doc/
@typedoc heredoc from both modules at this HEAD and at origin/main
leaves the two files byte-identical. The diff is 30 added lines and 0
removed lines across three files, every one of them inside a docstring or
README prose.

No new surface: grep for def register, def positions, :namespace
and namespace: across lib/ returns zero hits, here as on main.

Verification

• Full mix quality green on this HEAD: format, compile (warnings as
  errors), credo, dialyzer, deps, 482/482 tests, 95.7% coverage. Doctor,
  Gettext and Sobelow are the project's standing not-applicable skips.
• mix docs renders clean, and each of the three claims was grepped out of
  the generated StatifierPersistence.Storage.html and
  StatifierPersistence.Storage.InMemory.html - the acceptance criteria
  checked against the artefact a host actually reads, not against the
  source.
• No new tests, so no sabotage notes are due; the gate's sabotage scan
  reports nothing missing and nothing unverifiable.

No changelog fragment

changelog.d/README.md puts documentation on its "do not write a fragment
for" side, and nobody calling the public API can tell the difference: the
behaviour described is the behaviour that already shipped. Consistent with
the sp-a4x and sp-gb2 precedent, neither of which carried one.

Refs: sp-e8c

Closes sp-e8c