policy: Enforce TRUC by default and guard priority mining

[connorslab]

Default mempooltruc to enforce and remove its redundant corepolicy override. Explicit accept and reject settings remain available; other defaults and corepolicy overrides are unchanged.

Priority-space mining selects transactions individually without requiring fees. As reported in review, this can mine an ephemeral-dust parent without its child, including a parent stranded after the child is replaced. Exclude transactions with dust outputs from that selection path, using the mempool's configured dust relay rate. They remain eligible for fee-based package selection, and ordinary priority transactions remain eligible for priority space.

Update the help/docs and test the default and explicit TRUC modes with both corepolicy settings. Add a registered functional regression covering an underfunded mining package, a parent stranded by replacement of its child's sponsor spend, continued ordinary priority mining, and recovery/mining with a sufficiently funded child. The negative-modified-fee admission fix remains separate in #438.

Validation of 58313216fd8172324e3ef37039cefa08b63bf0e9: Linux build and test run passed.

• Release build, 18 targeted unit tests, and eight functional suites passed, including the existing coin-age priority and prioritisation tests.
• The new regression passed on this branch independently with the dust penalty disabled, and with policy: Allow negative modified fees for ephemeral dust #438's validation change applied with penalties both disabled and enabled.
• Removing the priority guard reproduced the expected assertion failure: the parent was selected despite its package not meeting the mining fee floor.
• Python syntax and git diff --check passed. The legacy-wallet variant was skipped because Berkeley DB was disabled; the GUI was not built.

Current commit d896460b810ca3a88dd305bc67c985b6ddacba2e differs from the tested commit only in the copyright attribution to the Bitcoin Knots developers. No executable code changed; tests were not rerun for this header correction. The PR is one squashed commit. Validation infrastructure is on a separate branch and is not part of this diff.

[kwsantiago]

Tested e2e with #438: anchors work on defaults. Merged together, though, a stranded zero-fee anchor parent can be mined via priority space.

[luke-jr]

Prefer not to update manpages or other generated files in PRs

[luke-jr]

What's the motivation for excluding ephemeral dust txs from priority space? I wonder if it would make more sense to enforce blockmintxfee to priority instead...?

[connorslab]

Superseded by #447 (TRUC enforcement by default) and #446 (apply the mining fee floor to priority selection). #438 remains the separate ephemeral-parent admission fix. For Ark-style child-funded anchor payments on native defaults, the relevant pair is #438 and #447; #446 addresses the separate priority-mining fee-floor issue. Closing this combined proposal in favor of those narrower PRs.