A forge's included credit counts what was spent, and home, app and pull pages show the same figure #11563

Part of #11541. On probe-hhmk the home said "The included credit is spent" after $0.48 of $2, the app page said $0.75 and "Spent $0.00", the pull said $0.48.

Done looks like:

• [ ] Find what the credit check counts (a per-run budget reservation such as claude: budget $5.00 larger than the credit would explain it) and make "spent" the sum of what runs actually cost; a reservation never reads as spent.
• [ ] One function gives the credit figure; the home notice (src/web/studio/crew_key.ts, src/tui/screens/crewkey.ts), the app page and the pull page all read it.
• [ ] A test with $2 credit and one $0.48 run: credit not spent, $1.52 left, the same number on all three pages' rendered text; a test where spending reaches $2 shows the spent notice.

Parked by the crew: the check stayed red through 2 automatic tries: check failed: stopped: used 18.0 cores for 30s, over its limit of 16; the busiest process is sh (pid 1920005, 500s of CPU) (pull #11572 closed; its branch scout/11563-a-forge-s-included-credit-counts-what-wa at ac156b0f48e0 keeps the work so far). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

try 1: src/spawn/credit_figure.ts, src/tui/screens/crewkey.ts, src/web/studio/app.ts, src/web/studio/change.ts, src/web/studio/crew_key.ts, test/credit-figure.… try 2: no files changed