refactor: drop the runtime u8 = int shim; u8 is typing-only

[JPHutchins]

Warning

LLM Disclosure

This PR was authored by claude-opus-5-5[1m] on behalf of @JPHutchins. They asked why #137 shimmed u8 rather than declaring types-bits[rt], approved the cleaner fix below, and asked for it as its own small PR.

Note

Targets screaming-goblin, per the epic's rule (intercreate/smpmgr#103).

#137's if TYPE_CHECKING: from types_bits import u8 / else: u8 = int meant the checkers saw Literal[0..255] while the interpreter saw int. This removes the else branch: u8 now exists only for the type checkers.

+from __future__ import annotations
 if TYPE_CHECKING:
     from types_bits import u8
-else:  # `types_bits` is typing-only; reaching its values at runtime needs its `rt` extra
-    u8 = int
-    return cast(Iterator[u8], itertools.cycle(range(0x100)))
+    return cast("Iterator[u8]", itertools.cycle(range(0x100)))

No new dependency, and no difference between what the checkers see and what runs.

Why this works, and why not types-bits[rt]

• Only two expressions evaluated u8 at runtime. One was the SMPClient.__init__ annotation (evaluated eagerly on 3.10–3.13); from __future__ import annotations now stores it as a string. The other was cast()'s first argument, which is an ordinary expression, not an annotation, so it is quoted.
• smp itself declares types-bits<0.3,>=0.2 without rt, so the library that defines u8 treats it as typing-only. The extra would only pull in annotated-types (a pydantic dependency, evicted in refactor(mcuboot): replace pydantic dataclasses with stdlib dataclasses #136) to evaluate objects nothing reads: cast() is a no-op and the annotation is never inspected.

Verification

• camas check: ruff, pydoclint, mypy, pyright, and tests all green.
• camas matrix: green on 3.10, 3.11, 3.12, 3.13, 3.14.
• A clean uv pip install . into a fresh 3.10 venv (what the transport-extras job does) has no annotated_types. import smpclient succeeds, wrapping_sequence() wraps 255 → 0, and u8 is unbound at runtime.

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