docs: design study for #83 — delegation and key mapping - #87

Discussion #86 captured this, but a discussion is easy to lose and hard to cite from
code review. misc/docs/ is where dol's durable design records live, so this lands
next to dol_issue18_design.md (its inverse) and dol_issue10_design.md.

Records, with the running-code evidence for each:

- TWO delegation routes, not one -- Store.__getattr__ AND
  DelegatedAttribute.__get__. A fix covering one is a silent no-op on the other,
  which is why several past fixes had to be applied twice.
- The 13-package census, stated accurately: the defect is overwhelmingly LATENT
  (it bites only when a user applies a key codec), and 12 survey claims were
  refuted outright. An earlier downstream draft claimed the family was actively
  destroying data; it is not, and the record says so.
- Options A-F with what each actually costs:
  A wrapped_self  -- has its own silent hole: degrades to the raw leaf when nothing
                     holds a strong reference to the wrapper, and where the leaf owns
                     a prefix the wrong answer is a plausible str. CPython 3.10-3.14.
                     Detectable via the KeysView probe, so it can be made loud.
  B declarative   -- dol already ships this (ingoing_key_methods), untested and
                     broken for leaf-defined methods. The reflective test is the part
                     that holds, not the registry.
  C free funcs    -- break on a non-Store layer, where the method form is right.
                     dol's own instance (content_url) had the bug until #85.
  D rebind        -- rejected in dol_issue18_design.md with evidence; binds to the
                     INNERMOST wrap, so it misses the Pipe case it exists to fix.
  E parallel maps -- broken as an ATTRIBUTE (a wrapper does not re-wrap one),
                     correct by construction as a sibling STORE keyed through
                     __getitem__. The only option needing no resolution primitive.
  F is-a          -- the terminal fix; dissolves #83, #18 and #6 together.

Section 5 is the explicit carry-forward list for a future redesign: the has-a/is-a
choice is one decision not three; a wrapper must be able to express "this method
takes a key"; the two routes must be unified; key mapping needs an inverse
(inward->outward has no supported helper); weakref backrefs are the wrong substrate
for correctness; non-Store layers exist in the wild; and "how many keyed methods does
this force adapters to write?" is a design metric worth tracking (azuredol has ~0 and
is clean).

Indexed from CLAUDE.md and dol_misc_docs_guide.md. All file:line citations verified
against the current source.