handle_hits, picks and graphics_pipeline's mine are searched on every draw
of the replay's render thread, the frame's bottleneck. std::unordered_map
paid a modulo by a prime and a node pointer chase per lookup; FlatMap
(open addressing, linear probing, at most half full) keeps key and value
in one array. HandleHit's tex_epoch takes texture_epoch()'s 32 bits, so a
slot is 56 bytes. tests/test_flat_map.py checks it against
std::unordered_map and texture_view_of over both.

Assisted-by: Claude Opus 5.5
Co-Authored-By: Claude Opus 5.5 <[email protected]>
(cherry picked from commit 0b8ad723bc42abbdf2c5be959089c1b412019222)

@@ -0,0 +38,4 @@
+    void clear() {
+        if (!count_) return;
+        count_ = 0;
+        if (slots_.size() > kKeptSlots) {   // (a cache cleared at its size limit: its memory goes back too)

@@ -1731,3 +1732,3 @@
     std::vector<uint32_t> handles;   // (at its answers' descriptors: their slots and the slots' versions)
 };
-thread_local std::unordered_map<uint64_t, Pick> picks;   // (each recording thread's: program_pick has its own lock)
+thread_local FlatMap<uint64_t, Pick> picks;   // (each recording thread's: program_pick has its own lock)