policy: Allow negative modified fees for ephemeral dust

[connorslab]

Allow the automatic sub-dust fee penalty on otherwise eligible ephemeral-dust transactions without permitting manual fee deltas.

Move the existing ephemeral fee precheck before the automatic dust-penalty deduction. At this point StageAddition has already applied the fee adjustments from mempool.mapDeltas, so the existing zero-base-fee and zero-modified-fee checks reject any nonzero net manual fee delta. The original precheck itself is unchanged; no duplicate penalty calculation or direct map access is needed.

Knots then applies its existing calculated dust penalty. A zero-value P2A output produces a -240-sat modified fee at 3 sat/vB dust relay, or -480 sats at 6 sat/vB. With the penalty disabled, it remains zero. The child must still cover the penalty and required package fees. Existing RPC policy continues to prohibit fee adjustments after a dust transaction enters the mempool.

No defaults, corepolicy overrides, TRUC rules, or dust-spending restrictions change. The intended version-3 parent/child anchor packages require -mempooltruc=enforce. The separate default-policy proposal is in #439.

Regression coverage tests two dust relay rates and disabled penalties, positive and negative pre-admission manual fee deltas, post-admission RPC rejection, the package fee boundary, child replacement, block-template inclusion and confirmation.

Validation of e155555218104959bb4172707b02ce2fd2413489: Linux build and targeted policy tests passed: the release build, 18 targeted unit tests, and five functional suites. The legacy-wallet variant was skipped because Berkeley DB was disabled; the GUI was not built. Python syntax and git diff --check passed. The validation workflow is on a separate branch and is not part of this PR.

[luke-jr]

The sub-dust penalty also directly conflicts with ephemeral anchor acceptance: it subtracts the anchor's dust shortfall from a zero-fee parent's modified fee, but ephemeral transactions must have both zero base fee and zero modified fee. The resulting negative modified fee causes rejection, even if the child pays sufficient fees.

Disabling it seems like the wrong solution. Why not just allow ephemeral anchors to have negative feerates?

[luke-jr]

Might make sense to drop the corepolicy overrides?

[kwsantiago]

tACK e77fe14. Please squash out the defaults change and its revert, and note that -mempooltruc=enforce is required.