infra(Tier 3): pre-signed bundle tx template pool

[0xfandom]

Context

Signing cost on the submission hot path is 1–5 ms of deterministic jitter. Currently cmd/executor/bundle.go constructs and signs each arbitrage transaction fresh per opportunity. Pre-signing templates for expected input-amount buckets removes this jitter and lets the executor ship whichever template matches the observed opportunity immediately.

This work is purely in-binary — no external service dependencies, no new credentials, no recurring cost.

Scope

Pre-signed transaction template pool

• New module: cmd/executor/template.go
• Template keyed by (token_pair, protocol_mix, amount_bucket)
• Amount buckets (tunable): 10, 25, 50, 100, 250, 500, 1000 WETH equivalent
• Signed at startup + rotation ticker (e.g., every N blocks or on nonce advance)
• Nonce pre-allocation window: reserve next K nonces for templates, track in cmd/executor/nonce.go
• Submitter picks closest template by (pair, protocol_mix, amount_bucket), mutates only slippage-independent fields (deadline, tipBps, minProfitOut)

Metrics

• aether_template_pool_size gauge
• aether_template_cache_hit_ratio gauge (rolling 1-minute window)
• aether_template_rotation_duration_ms histogram (rotation should not stall hot path)

Safety

• Template signing happens on a background goroutine, never on the arb-dispatch path
• If template cache misses (unusual pair or amount out of bucket), fall back to fresh signing path — no regression
• On reorg detection, invalidate templates with nonces ≤ last confirmed

Acceptance criteria

• Template pool rebuilds every N blocks without stalling hot path (measured: < 1 ms stall in aether_template_rotation_duration_ms)
• Cache-hit ratio ≥ 70 % after warmup on replay corpus of 100 historical arbs
• Unit tests for template selection, nonce rotation, reorg invalidation
• Bench: median submission time drops by ≥ 1 ms vs baseline
• No regression in gRPC test suite, Go test suite, contract test suite

Files touched (expected)

• cmd/executor/template.go (new)
• cmd/executor/nonce.go (expose reservation API)
• cmd/executor/submitter.go (template-path branch)
• cmd/executor/executor.go (wire template cache into submit flow)
• cmd/executor/template_test.go (new)

Out of scope

• L2 / non-Ethereum submit paths (explicitly not part of this effort)
• Private mempool integration (deferred, under discussion)