feat(rates): wire SEP-6 as Tier-3 fallback in fetchCorridorRates

[Johnalex-hub]

Summary

Implements B006 — wires SEP-6 as Tier-3 fallback in fetchCorridorRates after SEP-38 (Tier-1) and SEP-24 indicative (Tier-2) both fail.

• hasSep6(toml) checks capabilities.sep6 and TRANSFER_SERVER presence
• sep6IndicativeRate() calls getSep6Info() for fees then applies live FX rate from getUsdFxRate()
• Tier-3 only attempted when hasSep6(toml) is true — anchors without SEP-6 skip it cleanly
• Errors[] only populated when all three tiers fail; per-anchor Promise.all isolation preserved
• Adds sep6-fee to AnchorRate.source union in types/index.ts
• Cowrie now returns a rate on usdc-ngn instead of landing in errors[]

Test plan

• SEP-6-only anchor returns rate with source=sep6-fee when Tier-1 and Tier-2 fail
• totalReceived computed correctly from fees and FX rate
• errors[] empty when Tier-3 succeeds
• Anchor lands in errors[] with SEP-6 reason when all tiers fail
• Anchors without SEP-6 capability skip getSep6Info entirely
• 5 tests pass (npx vitest run tests/server-rates.sep6.spec.ts)

Closes #439

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[determined-001]

The commitlint check is failing — the commit subject line doesn't follow Conventional Commits format (e.g. length limit or missing type). Please amend the commit message to match the format (e.g. feat(rates): wire SEP-6 as Tier-3 fallback in fetchCorridorRates) and push.

[ezedike-evan]

The commitlint check is failing — the commit subject line exceeds the 72-character limit. Please shorten it (e.g. feat(rates): wire SEP-6 as Tier-3 fallback in fetchCorridorRates) and push so commitlint goes green.

[ezedike-evan]

The check (node 20) and check (node 22) jobs are now failing because main's server-rates.ts was refactored to use isAnchorAssetEnabled instead of USDC_ASSET. When CI runs against the merged ref, the import of USDC_ASSET gets dropped but the usage at line 165 remains, causing: error TS2304: Cannot find name 'USDC_ASSET'.

Please rebase this branch on top of main and update server-rates.ts to work with the current API in constants/anchors.ts (i.e. use isAnchorAssetEnabled / resolve the USDC_ASSET reference against the current export surface).

[ezedike-evan]

Thanks @Johnalex-hub for addressing the rebase conflict! The USDC_ASSET import is in place, sep6-fee is correctly wired into the QuotePill display, and all CI checks are green. The SEP-6 Tier-3 fallback in fetchCorridorRates looks clean — closes #439. Merging.