Tier C function JIT on by default, and a checkable roadmap

The whole-function JIT tier is enabled by default (parse 4.1× → 3.3× Node) and PERF_ROADMAP.md is written as a checkable path to V8 parity. Then the log goes quiet for six weeks.

4e12a406 and f0d725a8 enable the Tier C function-level JIT by default. 62dbba15 adds PERF_ROADMAP.md, whose tickets (B1, B2, B3…) number every later performance commit. The day before, 7e29d967 had fixed six JIT soundness bugs found by an adversarial audit of the session's own codegen. June ends with 1,039 commits.