Document profile comparison workflow

docs/internal/comparison-execution.md

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+# Comparison Execution Internals
+
+The comparison execution core receives an already prepared report and an
+already inspected, ordered profile list. It initializes an outcome for every
+selected profile, launches each started profile in its own goroutine, and
+waits for every started goroutine before returning. Results retain the supplied
+selection order even though execution completes in an arbitrary order.
+
+Every profile uses the exact inspected prompt identity and a private copy of
+the same prepared data package. Provider, generated-text validation, rendering,
+or debug-write failure becomes that profile's safe failed outcome and does not
+cancel its peers. The application deliberately imposes no additional semaphore:
+Promptkit owns backend capacity. Cancellation or a deadline marks unfinished
+outcomes as skipped or failed, joins work, and prevents bundle publication.
+
+When debugging is enabled, each execution receives a deterministic reference
+derived from the comparison identity, ordered profile position, and safe
+profile slug. This keeps concurrent captures separate. The debug writer itself
+owns secure-root validation and file permissions; operational retention and
+sensitivity are documented in the [operations guide](../operations.md).
+
+The output result and its safe errors are converted into the durable contract
+only by comparison publication. See [comparison publication
+internals](comparison-publication.md) and the external [comparison bundle
+contract](../integrations/comparison-bundle.md).