mempool: hand template selections the pool's own tx bodies

[average-gary]

MempoolHub::select_block_template deep-copied every selected transaction on each call (up to ~4 MB), though the pool already keeps each body as an Arc<Transaction>. It now returns those Arcs, so getblocktemplate and the SV2 Template Provider pay a pointer per transaction, and a template that outlives a transaction's eviction or replacement still holds its body. generate copies only for the block it mines.

No behavior change: same selection, same bytes, same JSON. Prerequisite for the SV2 TP fee-delta push (next PR), which keeps every template on the current tip per session and needs the bodies shared to keep that cheap.

Changes

• ActiveMempool::select_block_template / MempoolHub::select_block_template return Vec<(Arc<Transaction>, Selected)>; the RAM trade (a held body outlives eviction) is named in the rustdoc.
• Callers: GBT JSON and generate (rbitcoin-rpc), the SV2 template builder (Template.txs: Vec<Arc<Transaction>>), one journey assertion, two hub test asserts.
• Plan doc (docs/sv2-template-provider.md Plan A) and changelog.d/shared-template-txs.md.

Local verification

• fmt, clippy -D warnings, deny, ast-grep, ci-os-smoke.sh green.
• Full cargo test --workspace in Linux Docker (rust:1.95): 2726 passed, 0 failed.

🤖 Generated with Claude Code

[rearden-grok]

The selection change is sound. ActiveMempool::select_block_template hands out the pool's own Arc<Transaction> under the same lock as the graph walk, get_tx was already that map, and the callers that need an owned body (generate, solution assembly) still clone. Holding a template past eviction or replacement is the named RAM trade, and it is what a template has to do so RequestTransactionData and SubmitSolution survive a replacement.

Required checks are green. This is the right base for the fee-push stack.

Non-blocking: MempoolHub::select_block_template is the cross-crate surface and its rustdoc still only talks about fee and sigop metadata. A sentence there that the Arc keeps the body alive after eviction would match the note already on ActiveMempool.