fix(serve): close the resident-process staleness and growth holes

[iamtoruk]

Devil's-advocate pass over #956, fixing what survived scrutiny:

1. A memoized or burst-reused parse embeds costUSD priced under the config at parse time; the memo key now includes the model-alias, price-override, and local-model-savings hashes, so any pricing-affecting change is an automatic miss. (Config itself was already re-read per request via the preAction hook; the exposure was only the memo layer, bounded to seconds in practice, but the class is now closed structurally.)
2. RSS guard at 3GB: the resident child drops its in-memory memos instead of growing without bound on lifetime-scale corpora; next request re-parses once. No child exit, so the client's restart budget is untouched.
3. codeburn serve on a TTY prints what it is instead of silently blocking on stdin.

Attacks that did NOT survive (verified safe): config mutations reaching the child (preAction re-reads per request), orphaned children on app crash (stdin EOF ends the serve loop), request timeout mid-parse holding the refresh lock (#855 recovery handles the SIGKILL'd holder), option-state leakage (fresh program per request, pinned by test).

Full suite green; alias-hash covered by new tests; one-shot output unaffected (key widening only adds terms that are constant within a single run).