Updates stale doc passages that predate Position

[johnnyt]

Why

Two documentation passages predate landed work and mislead a host
following them (st-qmhx):

1. docs/durable-timers.md (Route B, process-less hosts) still claimed
   no MachineState serialization function exists in lib/ and pointed
   at st-m5c3 as the bead that owns the gap - but st-m5c3 is closed:
   Statifier.Position shipped to_binary/1 / from_binary/2
   (ADR-0052, ADR-0060).
2. docs/testing-charts.md showed {:statifier, "~> 2.0"} as the dep
   line, but nothing ships on Hex before 2.0.0 (ADR-0061).

What

• Route B's serialization paragraph now names
  Statifier.Position.to_binary/1 / from_binary/2, the deliberate
  nil routes / invoke_types on resume (ADR-0064), and defers to
  docs/persistence.md for the full recipe.
• The testing-charts dependency section now shows the SHA-pinned git
  dep from README.md per ADR-0061, with the Hex ~> 2.0 form noted as
  post-2.0.0.

Notes

• Docs only: no quality gate applicable to the diff. Full mix quality
  and mix quality --profile merge both ran green anyway; the ADR
  judge skipped (no files in a judged scope).
• PR Adds caller_context per ADR-0063 #215 also touches docs/durable-timers.md, but only the
  caller_context section (~lines 44-90), not the Route B passage
  changed here - no overlap.
• No changelog fragment per changelog.d/README.md (documentation is
  excluded).

Closes st-qmhx