Track and reset last avoided reachability separately

[themuffinator]

Motivation

• Restore Quake III retail behavior for avoid-reach tracking by tracking the most recent avoided reachability independently from the general avoid lists as described in docs/be_ai_parity_matrix.md.
• Ensure movement state stores the extra per-movestate fields required to record the last avoided reachability and its timing/tries.
• Make the reset helper BotResetLastAvoidReach operate on the tracked last-avoid entry rather than scanning only generic arrays, matching retail semantics.

Description

• Added fields lastavoidreach, lastavoidreachtime, and lastavoidreachtries to bot_movestate_t in src/botlib/ai_move/bot_move.h.
• Changed BotMove_ShouldAvoidReach to update the lastavoid* fields when a reachability is recognized as currently avoided and added a commented function header.
• Updated BotMove_ResetAvoidReach to also clear the lastavoid* fields, and reworked BotResetLastAvoidReach to clear the most-recently-tracked avoid entry by matching lastavoidreach into the per-state arrays and decrementing tries.
• Kept changes localized to src/botlib/ai_move/ and added brief commented headers to modified helpers for clarity.

Testing

• No automated tests were run.

---

Codex Task