`_redact_fn_cached` skipped memoization for any string over 16KB, so those
strings re-ran the full ~15-pass redactor on every single request. On a real
22MB session that is 59 large strings (29 unique, 0.76MB) costing 1.68s per
request — 99.9% of the recurring redaction cost once the small-string cache is
warm — repaid by every tab and every poll for a byte-identical result.
Large strings now get their own bounded cache, separate from the 4096-entry
small-string one so a large blob can never evict the thousands of small
recurring strings. Entry count and per-entry size are both capped; anything
above the ceiling stays uncached, keeping retained bytes bounded.
`_redact_value` additionally stops rebuilding containers whose descendants
were unchanged. Most of a transcript carries no credential marker, so the
unconditional dict/list comprehension deep-copied the whole payload to
reproduce an identical structure — pure CPU under the GIL, which serializes
concurrent tab loads on a threaded server. Callers treat redacted output as
read-only (`_public_message_projection` and `redact_session_data` both build
fresh containers), so sharing an unmodified subtree cannot leak a later
mutation back into session state. As soon as one string is masked, every
container on the path to it is rebuilt and the caller's original is untouched.
Measured on the live 22.6MB session, steady state: redact_session_data 3.78s
-> 1.96s single tab (-48%), 7.93s -> 4.06s with two concurrent tabs (-49%),
byte-identical output.
