[IDX-7032] Move to ticket-based eviction system when cache is full

[dalloriam]

Changes

• Changes the evict-on-insert from pessimistic to optimistic, using a ticket-based eviction mechanism (src)
  • Previous system expected the insert + evicts to fail, copied the eviction policy to non-destructively determine the set of keys to evict, and then performed the eviction
  • New system expects the insert + evicts to succeed, but keeps enough data to rollback if the insert fails (i.e. if a key to be evicted is preferred in cache over the key being inserted).
  • Even in worst-case scenarios, the new system performs less allocations and is overall faster than the old one.
• Fixes a bug where on_cache_hit was called twice on the eviction policy (src)
• Fixed iterator implementations + misc. refactor in GDSF (src)

[Copilot]

Pull request overview

This PR refactors cache eviction/constraint handling to a ticket-based flow: eviction policies now pop_victim() to produce a rollbackable ticket, and constraint policies return tickets (prepare_insert / prepare_replace) that become satisfied as evictions are registered. This lets the cache collect evictions without copying the whole constraint policy and supports aborting an insert by rolling back policy state.

Changes:

• Replace eviction-policy victim iteration APIs with pop_victim() + rollback(ticket) (LRU / SLRU / GDSF) and update tests accordingly.
• Introduce constraint “prepare” tickets for count/memory constraints and update cache insert/replace eviction collection logic to use them.
• Fix cache-hit event dispatch so hits are delivered once per policy, and add a regression test.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
tests/src/policy/eviction_segmented_lru_tests.cpp	Updates SLRU tests to use pop_victim() + rollback-based victim enumeration.
tests/src/policy/eviction_lru_tests.cpp	Updates LRU tests to use ticket-based victim popping/rollback.
tests/src/policy/eviction_gdsf_tests.cpp	Updates GDSF tests to use pop_victim() + rollback and validates victim ordering.
tests/src/policy/constraint_memory_tests.cpp	Updates memory-constraint tests to use prepare_insert/prepare_replace tickets.
tests/src/policy/constraint_count_tests.cpp	Updates count-constraint tests to use prepare_insert/prepare_replace tickets.
tests/src/memory_cache_tests.cpp	Adds regression test for replacement flow where the original key can be evicted mid-operation.
tests/src/event_dispatch_tests.cpp	Adds regression test ensuring cache-hit events are delivered once per policy.
tests/CMakeLists.txt	Adds the new event dispatch test file to the test target.
include/cachemere/policy/eviction_segmented_lru.h	Replaces victim iterators with eviction tickets and rollback.
include/cachemere/policy/eviction_lru.h	Replaces victim iterators with eviction tickets and rollback.
include/cachemere/policy/eviction_gdsf.h	Replaces victim iterators with eviction tickets and rollback for GDSF.
include/cachemere/policy/constraint_memory.h	Replaces can_add/can_replace with prepare tickets that accumulate freed memory via registered evictions.
include/cachemere/policy/constraint_count.h	Adds prepare tickets (but leaves can_add declared).
include/cachemere/cache.h	Implements ticket-based eviction collection, adds remove_popped_victim, and fixes cache-hit dispatch.

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