feat: loop widget backend API (#6608)

[aariya50]

Summary

• Backend-only looping orchestration for yield-leveraged strategies (deposit -> borrow/mint -> swap -> repeat)
• Three endpoints: GET /loop/bootstrap, POST /loop/execute, GET /loop/history
• Lending loop and CDP loop adapters with full swap integration
• Bootstrap includes carry model economics with swap impact adjustment

Closes #6608

[aariya50]

Loop Widget POC — Testnet Results

Yield-Bearing Assets

Token	Yield	Lending	CDP	USDST Pool	Loopable?
wstETH	4.09%	yes	yes	yes (46k)	yes
rETH	1.37%	yes	yes	no pool vs USDST	no
sUSDS	9.52%	no	no	yes	no — not collateral
syrupUSDC	5.15%	no	no	yes	no — not collateral

Only wstETH is loopable on testnet. rETH has no direct USDST swap pool. sUSDS and syrupUSDC are not configured as lending/CDP collateral.

Economics (ref $1k position, balanced risk)

Route	Net Carry	Swap Impact/Leg	Adjusted Carry
wstETH CDP (q=0.50, M=2x)	+5.58%	1.99%	+1.60%
wstETH Lending (q=0.55, M=2.2x)	+2.24%	1.99%	-0.75%

Formula: net = M*y - (M-1)*r - 2*(M-1)*fee, impact: g*dx/(x+g*dx).

Tx Fee Note

5 contract calls per iteration = 0.05 USDST. Covered by vouchers if available. 2-loop = 0.10 USDST.

wstETH CDP Loop x2 (positive carry)

	Expected	Actual
Flow	deposit → mint → swap → repeat	6/6 txs
Collateral (from 1.0 wstETH)	~1.6x	1.613 wstETH ($4,259)
Debt	~$2,600	2,610 USDST
CR	>155%	163%
Adjusted carry	+1.60%	~+1.95% (better than estimate — actual impact < modeled)

wstETH Lending Loop x2 (negative carry)

	Expected	Actual
Flow	supply → borrow → swap → repeat	6/6 txs
Adjusted carry	-0.75%	Confirmed negative (5% borrow > 4.09% yield at leverage)