fix: add iteration limit to SPV fee calculation loop

[thepastaclaw]

Replace unbounded loop with a capped for loop (max 50 iterations) in the SPV fee calculation to prevent potential infinite loops.

Problem

The fee estimation loop in build_transaction_for_spv_wallet uses an unbounded loop that adjusts a scale factor to find a valid transaction. If the scale factor converges without meeting either exit condition (successful build or InsufficientFunds with no progress), the loop runs forever.

Fix

Cap the loop at 50 iterations and return a clear error if no valid transaction is found within that budget. 50 iterations is generous — in practice, convergence happens within 2-3 iterations.

Cherry-picked from ralph/improvements (commit 557acb2).

Validation

What was tested:

• cargo clippy --all-features --all-targets -- -D warnings — lint check
• cargo test --all-features --workspace — full workspace test suite

Results:

• All local commands passed
• Clippy CI check — pass (5m27s)
• Test Suite CI check — pass (7m1s)

Environment: Local macOS arm64; GitHub Actions CI (ubuntu-latest)

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[thepastaclaw]

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