Fix Codex borrow observations and cancellation

[HTExplicit]

The first live acceptance of the unified console exposed two observation errors: prepared HTTP forwarding restored the caller context and lost the transient borrow marker, and read-ahead cancellation after a valid completed stream made diagnostics report failure. The original HTTP responses contained completed terminals and visible OK output.

• Derive HTTP borrow application from the immutable preparation proof and the actual outgoing cookie; use the same check for the usage table and diagnostic capture.
• Snapshot captures under a lock. A successful sender plus a parsed completed terminal and visible output determines diagnostic completion; retain trailing read cleanup details separately as read_error. Missing terminals, upstream errors, and incomplete output still fail.
• Protect usage observation during read/close and preserve the first terminal event, including when cleanup later cancels the reader.
• Check the task parent before and after execution admission. Cancellation can release one child's permit before reaching a queued sibling; that sibling must not start a request while its own cancellation signal is still propagating.

Validation: regression coverage passes for completed streams with trailing cancellation, missing terminal events, actual borrowed-cookie counters, changed/missing cookies, and terminal ordering. Existing failed/incomplete Pelican cases still pass. The queued/in-flight cancellation regression and a deterministic parent-cancellation propagation-gap case pass without weakening the original no-dispatch assertion. Vue typecheck and focused lint passed; the CI race selection now includes the HTTP completion/dispatch regression. The existing production diagnostic ledger has used 9 of the authorized maximum 20 requests; only affected HTTP combinations will be rechecked after deployment, with the remaining budget enforced.