fix: enforce on-chain fallback 60s cap for new markets (H2)

[0x-SquidSol]

Summary

• The M-4 on-chain fallback had a 60s time cap to prevent pushing stale prices indefinitely
• The guard lastExternal > 0 was bypassed for any market that never received an external price (DexScreener/Jupiter), because lastExternalPriceMs defaulted to 0
• A manipulated on-chain authorityPriceE6 would be pushed forever for new/unlisted markets

Fix

Seed lastExternalPriceMs to Date.now() on first encounter of each market:

• New markets get a 60s grace window from their first fallback attempt
• After 60s with no external price, the oracle goes stale naturally (as M-4 intended)
• Remove the lastExternal > 0 guard since lastExternal is now always a real timestamp
• When an external price is eventually fetched, the clock resets normally

Test plan

• All 133 existing tests pass
• Fix is a 6-line change with clear semantics

🤖 Generated with Claude Code

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Configuration used: defaults

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Run ID: 9a347472-669e-45dd-8cdb-ba9a7c3634e7

📥 Commits

Reviewing files that changed from the base of the PR and between 63ad1a0 and 48930fd.

📒 Files selected for processing (1)

• src/services/oracle.ts

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