Retain anchor reserves when sending all funds

[matbalez]

Fixes #40.

Summary

• retain the Anchor-channel reserve when sending all on-chain funds
• update the gRPC, CLI, and MCP descriptions to match the reserve-retaining behavior
• add a real-bitcoind end-to-end regression test proving spendable funds drain while the nonzero Anchor reserve remains

Validation

• focused end-to-end test against real bitcoind
• root workspace tests (121 passed)
• root workspace strict Clippy with all features
• standalone e2e strict Clippy
• formatting and git diff --check

[ldk-reviews-bot]

I've assigned @wpaulino as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[benthecarman]

can you have it squash? then looks good

[matbalez]

Squashed the two PR commits into a single commit: e38a9df (Retain anchor reserves when sending all funds). The squashed commit preserves the exact reviewed tree from 1040f1a, so there were no code or content changes during the history rewrite. The required Co-authored-by and Signed-off-by trailers are also retained. CI was re-triggered on the squashed head.