Adds tsconfig.json (strict) over product code, fixes its remaining 90
diagnostics (type-only, interface corrections, and explicit narrowing),
and adds a typecheck job to the required free-tests aggregate running
bun run typecheck, the test-code ratchet (identity -> count baseline, fails
on new, repeated, or unlocked fixed diagnostics), and the lib/cso format
check. Reuses fixes from #2447 where they still applied.
Mechanical reformat only. Minified transpile output is byte-identical for
21 of 22 files; witness.ts differs only in three regex flag orders
(/mi -> /im), which JavaScript canonicalizes. Source-text assertions over
lib/cso now compare whitespace-insensitively with the same tokens.
garrytan