refactor(p2p): rewrite FileStoreTxCollection with retry, backoff, and shared worker pool

[PhilWindle]

Summary

• Rewrites FileStoreTxCollection with a shared worker pool, deadline-based expiry, and retry with round-robin and exponential backoff
• Splits into two instances (fast/slow) with configurable worker counts and backoff settings via 6 new config fields
• Extracts getProofDeadlineForSlot as a public free function from SlowTxCollection for reuse
• Workers pull entries with priority (fewest attempts first), skip entries in backoff, and remove expired entries

Test plan

• All 10 file_store_tx_collection.test.ts tests pass (updated for new interface, added retry/backoff/deadline/clearPending tests)
• All 22 tx_collection.test.ts tests pass (updated for two file store instances)
• yarn build succeeds, yarn format p2p and yarn lint p2p clean
• Tests use deterministic waits (waitForSourceCalls, waitForTxsAdded) instead of blind sleeps to avoid flakiness

🤖 Generated with Claude Code

[AztecBot]

Flakey Tests

🤖 says: This CI run detected 3 tests that failed, but were tolerated due to a .test_patterns.yml entry.

\033FLAKED\033 (8;;http://ci.aztec-labs.com/40febf05b578eedb�40febf05b578eedb8;;�):  yarn-project/end-to-end/scripts/run_test.sh simple src/e2e_epochs/epochs_mbps.parallel.test.ts "builds multiple blocks per slot with transactions anchored to proposed blocks" (230s) (code: 1) group:e2e-p2p-epoch-flakes
\033FLAKED\033 (8;;http://ci.aztec-labs.com/561d31cf1057a897�561d31cf1057a8978;;�): yarn-project/end-to-end/scripts/run_test.sh ha src/composed/ha/e2e_ha_full.test.ts (121s) (code: 1)
\033FLAKED\033 (8;;http://ci.aztec-labs.com/b4a013e9893fcfb1�b4a013e9893fcfb18;;�):  yarn-project/end-to-end/scripts/run_test.sh simple src/e2e_p2p/gossip_network.test.ts (380s) (code: 1) group:e2e-p2p-epoch-flakes

[alexghr]

We reviewed the code. It looks fine. We'll try to combine the code dealing with pickNextEntry and the one dealing with getBackOffMs into one function