easy timeouts, shrink expire timer indices

[icing]

The enum exire_id fits easily into a uint8_t. Use that type for storing expire timeout list indices.

[Copilot]

Pull request overview

This PR reduces the storage size of per-easy-handle timeout list indices by changing expire_timers.next[] and expire_timers.first from expire_id to uint8_t, since EXPIRE_LAST fits in a byte. This helps shrink struct expire_timers and the per-handle timeout bookkeeping footprint in the multi interface.

Changes:

• Change struct expire_timers list linkage fields (next[], first) to uint8_t.
• Update timeout list iteration and validation logic in lib/multi.c to use uint8_t indices.
• Adjust timer tracing iteration in lib/curl_trc.c and parameter naming in lib/multiif.h.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
lib/urldata.h	Shrinks timeout list linkage storage to uint8_t.
lib/multiif.h	Renames Curl_expire_clear parameter for consistency (id → eid).
lib/multi.c	Updates timeout list walk/insert/remove code for byte-sized indices.
lib/curl_trc.c	Updates verbose timer tracing loop variable to uint8_t.

Suppressed comments (2)

lib/multi.c:3712

• timeouts->next is now a uint8_t indexed by the expire id, but this code still writes to timeouts->next[eid] (and does it twice). If an out-of-range expire_id is ever passed, the current cast-based check can wrap and this becomes an out-of-bounds write. Use a range check on eid before casting, and index next[] via the validated uint8_t id, with a single assignment.

  }
  timeouts->next[eid] = *anchor;
  timeouts->next[eid] = *anchor;
  *anchor = id;

lib/multi.c:3737

• Curl_expire() performs its argument validation by first narrowing eid to uint8_t and then comparing. That allows values > 255 to wrap and evade the check. Validate eid before any narrowing (or just validate eid directly here, since the uint8_t copy is otherwise unused).

  uint8_t prev_id = timeouts->first, id = (uint8_t)eid;
  struct curltime set;

  if(id >= EXPIRE_LAST) {
    DEBUGASSERT(0);

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