Diagnostic projection policy for analysis sessions whose results are private.

A session over private records may not forward evaluator-produced message text: that text can quote a captured record, and a host may route diagnostics somewhere the result itself never goes. It may still tell the operator what went wrong, because the fault often describes nothing but the operator's own submitted source — or, for a narrower class, nothing private that this evaluation itself captured.

Three admission rules apply, all rebuild-or-bound, never raw forward of untrusted evaluator prose outside their footing:

1. Source-derived structured detail (today :unbound_var). A message is rebuilt only when every name appears verbatim in the submitted source. Every other byte is a literal in this module.

2. Allowlisted compile/analyze kinds with no capability activity. Several of these kinds (:invalid_arity, :invalid_form, :unknown_tool, :private_tool_unauthorized) also have runtime constructors. Admission therefore requires details.capability_activity? == false — measured for this evaluation — and a byte/UTF-8 bound at this boundary. The allowlist names which kinds may be considered; the activity flag is the load-bearing gate. Messages are a function of the submitted source, prelude surface, and installed tool names, not of values read from private records.

3. Code-owned structured selectors with no capability activity. The private prelude boundary may replace an evaluator message with a closed :safe_diagnostic shape. This module admits only exact selector shapes it knows and emits only fixed literals. The current selector explains that analysis/counters expects a named-argument map and uses run_id in its remediation example; arbitrary :invalid_tool_args messages never enter the kind allowlist above.

details is evaluator output and remains untrusted even when it contains a code-owned selector: it can only select among exact fixed shapes and never contributes rendered bytes. Anything outside those rules collapses to redacted_message/0.