Read per-pool AMM fees from the venue instead of assuming a constant

[Naajih09]

Closes #298

What this does

Pricing previously used a single fee constant for every AMM pool. Protocols set fees per pool, so pools with a different fee were priced wrongly with no error. This change reads each pool's fee from the venue, caches it, and uses it wherever a fee is needed.

• Reads the fee for each AMM pool from the venue.
• Caches the fee and tests the cache, including expiry and refresh.
• Excludes any pool whose fee cannot be read from execution estimates, and reports it in the response so the exclusion is visible.
• Removes the fee constant from the pricing path.
• Echoes network in every response it touches and treats ?network= as required on any new behavior, per the issue's guidance.

Provenance

Responses that use a pool's fee include the pool ID, the fee in basis points, and when that fee was read. Excluded pools are listed with the reason.

Scope

SDEX order-book levels do not have a per-pool fee, so this PR covers AMM pools only. Any SDEX fee handling stays unchanged.

Known limits

• Fee source. I have not yet confirmed the exact field the venue exposes for each pool's fee. This needs to be checked against the protocol's documentation before merge.
• Cache lifetime. The cache duration is a judgment call. I'll state the value used and why in the PR review.
• Not run against a live service. Tests run locally with fixtures. Verification against live pools is still needed.

How to verify

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