[ty] Improve flow snapshot performance (#26012)
## Summary
When we build a semantic index, we currently snapshot flow state by
eagerly cloning every symbol and member's bindings and declarations at
control-flow branches. We also apply scope-wide reachability constraints
to every live place immediately, even though most places are unchanged
before the branches merge.
We now represent snapshots with copy-on-write shared place states and
record scope-wide reachability constraints in an append-only
parent-linked structure. A place materializes its pending constraints
only when it is read or mutated. When both branches still share the same
place state, we merge only their path constraints, allowing
complementary truthy and falsy paths to cancel without rebuilding the
place.
The specialized star-import snapshot path remains narrowly scoped and
avoids allocating a temporary member list.
## Performance
The current [CodSpeed
comparison](https://github.com/astral-sh/ruff/pull/26012#issuecomment-4709947650)
reports a 6.78% overall improvement, with 12 improved benchmarks and no
regressions. DateType improved by 15.51%, anyio by 9.10%, and attrs by
6.96%. CodSpeed notes that some comparisons used different runtime
environments, which may affect the exact percentages.
The typing-conformance and ecosystem comparisons reported no diagnostic
changes, and retained memory was effectively unchanged.
