migrate to ty

Mirrors the sister faststream-concurrent-aiokafka project's planning
scaffold and design pattern. PR1 will tighten the upper bound to <0.7,
land the pending build-backend swap and coverage gate, and create
CLAUDE.md. PR2 will bump to 0.7 and drop the _OLD_MIDDLEWARES compat
shim entirely (single code path, no <0.7 support).

Spec: planning/specs/2026-06-04-faststream-0.7-migration-design.md

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

…LAUDE.md

Three independently-motivated changes bundled per the migration design
(planning/specs/2026-06-04-faststream-0.7-migration-design.md):

1. Tighten the faststream dependency from "faststream>=0.5,<1" to
   "faststream>=0.5,<0.7". Guards against the silent pull of 0.7 the
   previous <1 bound allowed. Companion PR (chore/faststream-0.7-migration)
   bumps to >=0.7,<0.8 and drops <0.7 support entirely.

2. Swap build backend from hatchling to uv_build. Matches the toolchain
   used across the modern-python sister projects.

3. Adopt --cov-fail-under=100 in pytest addopts. Hardens the coverage
   gate before the migration PR disturbs anything.

Also creates CLAUDE.md with the standard ## Workflow section so future
AI sessions on this repo get the same brief as the sister projects.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

chore: pin faststream <0.7, swap build backend, adopt cov gate, add CLAUDE.md