docs: record Rust anonymous principal checks

[rmcdaniel]

Purpose

Describe the executable Rust anonymous-actor phase in
durable-workflow/sample-app#174.
Advances #122.

The final isolated phase follows every named-credential assertion. It verifies
token-driver refusal before explicitly recreating the disposable frontend with
the documented none driver. Actual Rust callers and worker send no credentials
while the gateway injects the same forged identity matrix. Starts, signals,
completion, queries, authored failure and terminal cancellation inspect original
histories and server-controlled audit actors. Operator visibility and
cooperative attribution remain separate cases.

Merge dependency

Sample App #174 merges as 8ef3332a23d1f210941d98ee73c64873a45255a8 after
all 15 normal gates and the
published execution
pass. Actual runner 6c3e45265ebf00b5251339ebd37d2889242acab2 executes the
qualified head's identical tree. The anonymous phase proves token-driver refusal,
12 successful credential-free injected requests, server/anonymous actors,
unchanged query history and original terminal boundaries. Existing named actor,
rotation and physical cold-recovery cases also pass, for 11 original runs and
48 successful requests overall. Twenty-four focused checks and this PR's shared
contract checks pass. Hosted artifact 11588483699 retains raw histories and
receipts for 30 days.

No package, image or release change is needed for these inventory edits.