refactor: adopt alias __MODULE__ for struct references

[nyo16]

Stacked on # — merge that first; this diff is the
single commit a97e8c8.

What

Adopts the alias __MODULE__ idiom for struct references across the library:
%__MODULE__{...} builds, pattern matches, update syntax, and @type t definitions
become the module's short name (%Message{}, %Context{}, …). 37 files, ~253 refs,
matching the existing exemplar in memory/store/hybrid.ex.

Several moduledocs already wrote examples with the short name (Registry.new(),
Agent.new(...)) — the alias makes those docs literally accurate.

Scope

• Converted: struct references only
• Untouched: process-identity uses (GenServer.start_link(__MODULE__, ...),
  :ets.new(__MODULE__, ...), @behaviour, callback-module arguments)
• Excluded: errors/base.ex (refs inside quote do — macro hygiene),
  errors.ex (9 submodules × 1 ref — alias noise), four 1-ref files

Notes for review

• Multi-module files got one alias per struct module: eval/result.ex
  (Result + SuiteResult) and eval/metrics.ex (Metrics + Metrics.Summary)
• Stdlib-shadowing candidates (Registry, Agent, Node) were audited per file —
  no stdlib calls to those names exist in the converted files
• skill.ex's two refs verified to sit outside its __using__ quote block

Verification

Zero behavior change — alias is compile-time lexical.

• mix test: 1992 passed (7 doctests), 102 excluded — exact baseline match
• mix compile --warnings-as-errors, mix format --check-formatted,
  mix credo --strict (0 issues), mix docs warning-free
• Residual grep -rn "%__MODULE__{" lib shows only the 15 documented exclusion refs