[nofuturekid]

Fixes #151

What changed

Camera::answer read selector 2 straight from the device, so no test reached its arms. The poll body moved into poll_answer(command, seq, gap, read) in crates/obsbot-device/src/family/exchange.rs; read is a closure returning one selector-2 read, so a test can script the replies. Camera::answer calls it with REPLY_GAP (50 ms) and || self.dev.xu_get(2): same sleep before every read, same 20 attempts, same read, nothing sent. Production code otherwise unchanged.

The seam is deliberately small (one function, one closure) so that #152 (stacked on this branch) can use it.

New tests (module exchange::tests):

• a rejected answer returns Error::Rejected(command, code) (code -3) after exactly one read
• an empty answer (type 0x09 echoing the command) returns an empty list after one read
• stale replies (other sequence) run through all 20 reads and end in Error::NoAnswer(command)
• two failed reads, then a value: the poll goes on and returns the value (3 reads)

How it was verified

Gate, on this branch: cargo fmt --check, cargo clippy --all-targets --locked -- -D warnings, cargo test --locked: all green. tests/byte_guard.rs and tests/cli_surface.rs pass with their fixtures unchanged.

Mutations by hand in poll_answer (each reverted afterwards; the file is identical to the committed one):

Mutation	Result
Some(Answer::Rejected(code)) => return Err(...) replaced by Some(Answer::Rejected(_)) => {} (the issue's mutant)	a_rejected_answer_returns_the_cameras_code_at_the_first_poll FAILED
rejected error built with code 0	same test FAILED
Answer::Empty arm replaced by {}	an_empty_answer_is_an_empty_list_not_a_missing_one FAILED
failed read ends the poll (read()?)	a_failed_read_is_polled_again_and_a_later_value_is_returned FAILED
REPLY_ATTEMPTS 20 to 19	without_a_matching_answer_the_poll_gives_up_after_all_attempts FAILED

With the current code all four tests pass. No CHANGELOG line: tests only, nothing a user can see. No camera needed; the tests open no device.

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