tick decode: step over a tick too wide to read, rather than dropping the message (ibx#302)

[userFRM]

Summary

• An extended 35=P entry states its width in a full byte, so it can name a value wider than this decoder returns. The decoder abandoned the whole message at that point, so every tick after it went with it — including the other server tags in the same message.
• The field is now stepped over and decoding continues. The entry itself is still dropped rather than guessed at, and a genuinely truncated message still ends where it did.

Why

A quote update sitting behind one of these simply never arrived, and nothing recorded that it had. One entry the decoder cannot represent should cost that entry, not the rest of the message.

Closes #302.

Notes

Stepping over needs a skip on the bit reader. Its width comes off the wire, so it is bounded by what is actually left rather than by an addition that wraps in release and rewinds the reader instead of stopping it. It is private — only this decoder steps.

The test payload builder gained the ability to emit a field wider than a u64, which is what building one of these requires.

Test plan

• cargo test --offline --lib — 804 passed. The 2 failures are config::expiry_tests::{named_zone_converts_with_dst, instant_round_trips_to_wire}, which fail on the base commit too (fixed separately in config: resolve the legacy timezone names IB states its times in (ibx#335) #336).
• cargo check --offline clean on every target: --lib, --features python, --bins, --examples, and each of the eight tests/*.rs targets individually.
• tests/ib_paper_compat unchanged from base — identical sorted diagnostic sets, not merely equal counts.
• Mutation check: restoring the bare return fails a_tick_too_wide_to_read_does_not_discard_the_rest_of_the_message; making skip add unchecked fails skipping_past_the_end_refuses_rather_than_wrapping. Both by name.
• The test asserts the ticks after the wide entry and the whole server tag after it, so it distinguishes "kept going" from "stopped one entry later".

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